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/agents/builtin-agents/model-resolution.ts

@@ -1,4 +1,5 @@
 import { resolveModelPipeline } from "../../shared"
+import { transformModelForProvider } from "../../shared/provider-model-id-transform"
 
 export function applyModelResolution(input: {
   uiSelectedModel?: string
@@ -20,8 +21,10 @@ export function getFirstFallbackModel(requirement?: {
 }) {
   const entry = requirement?.fallbackChain?.[0]
   if (!entry || entry.providers.length === 0) return undefined
+  const provider = entry.providers[0]
+  const transformedModel = transformModelForProvider(provider, entry.model)
   return {
-    model: `${entry.providers[0]}/${entry.model}`,
+    model: `${provider}/${transformedModel}`,
     provenance: "provider-fallback" as const,
     variant: entry.variant,
   }