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/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
 }