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/tools/delegate-task/model-selection.ts

@@ -1,5 +1,6 @@
 import type { FallbackEntry } from "../../shared/model-requirements"
 import { fuzzyMatchModel } from "../../shared/model-availability"
+import { transformModelForProvider } from "../../shared/provider-model-id-transform"
 
 function normalizeModel(model?: string): string | undefined {
   const trimmed = model?.trim()
@@ -38,7 +39,8 @@ export function resolveModelForDelegateTask(input: {
       const first = fallbackChain[0]
       const provider = first?.providers?.[0]
       if (provider) {
-        return { model: `${provider}/${first.model}`, variant: first.variant }
+        const transformedModelId = transformModelForProvider(provider, first.model)
+        return { model: `${provider}/${transformedModelId}`, variant: first.variant }
       }
     } else {
       for (const entry of fallbackChain) {