fix: add google provider model transform across all resolution paths

transformModelForProvider only handled github-copilot provider, leaving
google provider models untransformed. This caused ProviderModelNotFoundError
when google/gemini-3-flash was sent to the API (correct ID is
gemini-3-flash-preview).

Changes:
- Add google provider to transformModelForProvider with idempotent regex
  negative lookahead to prevent double -preview suffix
- Fix category-default path in model-resolution-pipeline when
  availableModels is empty but connected provider exists
- Fix getFirstFallbackModel first-run path that constructed raw model IDs
  without transformation
- Fix github-copilot provider gemini transforms to also use idempotent
  regex (was vulnerable to double-transform)
- Extract transformModelForProvider to shared module (single source of
  truth, imported by cli and shared layers)
- Add 20 new test cases: unit tests for both providers, runtime
  integration tests for category-default and fallback-chain paths,
  double-transform prevention for both providers

src/shared/model-resolution-pipeline.ts

@@ -2,6 +2,7 @@ import { log } from "./logger"
 import * as connectedProvidersCache from "./connected-providers-cache"
 import { fuzzyMatchModel } from "./model-availability"
 import type { FallbackEntry } from "./model-requirements"
+import { transformModelForProvider } from "./provider-model-id-transform"
 
 export type ModelResolutionRequest = {
   intent?: {
@@ -85,10 +86,13 @@ export function resolveModelPipeline(
       if (parts.length >= 2) {
         const provider = parts[0]
         if (connectedProviders.includes(provider)) {
+          const modelName = parts.slice(1).join("/")
+          const transformedModel = `${provider}/${transformModelForProvider(provider, modelName)}`
           log("Model resolved via category default (connected provider)", {
-            model: normalizedCategoryDefault,
+            model: transformedModel,
+            original: normalizedCategoryDefault,
           })
-          return { model: normalizedCategoryDefault, provenance: "category-default", attempted }
+          return { model: transformedModel, provenance: "category-default", attempted }
         }
       }
     }
@@ -108,10 +112,11 @@ export function resolveModelPipeline(
         for (const entry of fallbackChain) {
           for (const provider of entry.providers) {
             if (connectedSet.has(provider)) {
-              const model = `${provider}/${entry.model}`
+              const transformedModelId = transformModelForProvider(provider, entry.model)
+              const model = `${provider}/${transformedModelId}`
               log("Model resolved via fallback chain (connected provider)", {
                 provider,
-                model: entry.model,
+                model: transformedModelId,
                 variant: entry.variant,
               })
               return {

src/shared/model-resolver.test.ts

@@ -543,7 +543,8 @@ describe("resolveModelWithFallback", () => {
       const result = resolveModelWithFallback(input)
 
       // then - should use github-copilot (second provider) since google not connected
-      expect(result!.model).toBe("github-copilot/gemini-3-pro")
+      // model name is transformed to preview variant for github-copilot provider
+      expect(result!.model).toBe("github-copilot/gemini-3-pro-preview")
       expect(result!.source).toBe("provider-fallback")
       cacheSpy.mockRestore()
     })
@@ -795,8 +796,82 @@ describe("resolveModelWithFallback", () => {
       // when
       const result = resolveModelWithFallback(input)
 
-      // then - should use categoryDefaultModel since google is connected
-      expect(result!.model).toBe("google/gemini-3-pro")
+      // then - should use transformed categoryDefaultModel since google is connected
+      expect(result!.model).toBe("google/gemini-3-pro-preview")
+      expect(result!.source).toBe("category-default")
+      cacheSpy.mockRestore()
+    })
+
+    test("transforms gemini-3-flash in categoryDefaultModel for google connected provider", () => {
+      // given - google connected, category default uses gemini-3-flash
+      const cacheSpy = spyOn(connectedProvidersCache, "readConnectedProvidersCache").mockReturnValue(["google"])
+      const input: ExtendedModelResolutionInput = {
+        categoryDefaultModel: "google/gemini-3-flash",
+        availableModels: new Set(),
+        systemDefaultModel: "anthropic/claude-sonnet-4-5",
+      }
+
+      // when
+      const result = resolveModelWithFallback(input)
+
+      // then - gemini-3-flash should be transformed to gemini-3-flash-preview
+      expect(result!.model).toBe("google/gemini-3-flash-preview")
+      expect(result!.source).toBe("category-default")
+      cacheSpy.mockRestore()
+    })
+
+    test("does not double-transform categoryDefaultModel already containing -preview", () => {
+      // given - category default already has -preview suffix
+      const cacheSpy = spyOn(connectedProvidersCache, "readConnectedProvidersCache").mockReturnValue(["google"])
+      const input: ExtendedModelResolutionInput = {
+        categoryDefaultModel: "google/gemini-3-pro-preview",
+        availableModels: new Set(),
+        systemDefaultModel: "anthropic/claude-sonnet-4-5",
+      }
+
+      // when
+      const result = resolveModelWithFallback(input)
+
+      // then - should NOT become gemini-3-pro-preview-preview
+      expect(result!.model).toBe("google/gemini-3-pro-preview")
+      expect(result!.source).toBe("category-default")
+      cacheSpy.mockRestore()
+    })
+
+    test("transforms gemini-3-pro in fallback chain for google connected provider", () => {
+      // given - google connected, fallback chain has gemini-3-pro
+      const cacheSpy = spyOn(connectedProvidersCache, "readConnectedProvidersCache").mockReturnValue(["google"])
+      const input: ExtendedModelResolutionInput = {
+        fallbackChain: [
+          { providers: ["google", "github-copilot"], model: "gemini-3-pro" },
+        ],
+        availableModels: new Set(),
+        systemDefaultModel: "anthropic/claude-sonnet-4-5",
+      }
+
+      // when
+      const result = resolveModelWithFallback(input)
+
+      // then - should transform to preview variant for google provider
+      expect(result!.model).toBe("google/gemini-3-pro-preview")
+      expect(result!.source).toBe("provider-fallback")
+      cacheSpy.mockRestore()
+    })
+
+    test("passes through non-gemini-3 models for google connected provider", () => {
+      // given - google connected, category default uses gemini-2.5-flash (no transform needed)
+      const cacheSpy = spyOn(connectedProvidersCache, "readConnectedProvidersCache").mockReturnValue(["google"])
+      const input: ExtendedModelResolutionInput = {
+        categoryDefaultModel: "google/gemini-2.5-flash",
+        availableModels: new Set(),
+        systemDefaultModel: "anthropic/claude-sonnet-4-5",
+      }
+
+      // when
+      const result = resolveModelWithFallback(input)
+
+      // then - should pass through unchanged
+      expect(result!.model).toBe("google/gemini-2.5-flash")
       expect(result!.source).toBe("category-default")
       cacheSpy.mockRestore()
     })

src/shared/provider-model-id-transform.ts

@@ -6,16 +6,13 @@ export function transformModelForProvider(provider: string, model: string): stri
 			.replace("claude-sonnet-4-5", "claude-sonnet-4.5")
 			.replace("claude-haiku-4-5", "claude-haiku-4.5")
 			.replace("claude-sonnet-4", "claude-sonnet-4")
-			.replace("gemini-3-pro", "gemini-3-pro-preview")
-			.replace("gemini-3-flash", "gemini-3-flash-preview")
+			.replace(/gemini-3-pro(?!-)/g, "gemini-3-pro-preview")
+			.replace(/gemini-3-flash(?!-)/g, "gemini-3-flash-preview")
 	}
 	if (provider === "google") {
-		if (!model.endsWith("-preview")) {
-			return model
-				.replace("gemini-3-pro", "gemini-3-pro-preview")
-				.replace("gemini-3-flash", "gemini-3-flash-preview")
-		}
 		return model
+			.replace(/gemini-3-pro(?!-)/g, "gemini-3-pro-preview")
+			.replace(/gemini-3-flash(?!-)/g, "gemini-3-flash-preview")
 	}
 	return model
 }