fix(model-fallback): apply transformModelForProvider in getNextFallback

[east-shine]

Summary

getNextFallback() in the model-fallback hook returns raw model names from the hardcoded fallback chain without calling transformModelForProvider(). This causes fallback to fail silently for providers that require different model name formats:

• github-copilot: expects dot notation (claude-sonnet-4.6), but the chain stores hyphens (claude-sonnet-4-6)
• google: expects -preview suffix (gemini-3-pro-preview), but the chain stores plain names (gemini-3-pro)

Root Cause

The background-agent retry handler (src/features/background-agent/fallback-retry-handler.ts:94) correctly calls transformModelForProvider() before returning the fallback model. However, the sync chat.message hook in model-fallback/hook.ts was missing this call — a copy-paste omission.

Before (broken):

return {
  providerID,
  modelID: fallback.model,  // raw name → github-copilot and google reject it
  variant: fallback.variant,
}

After (fixed):

return {
  providerID,
  modelID: transformModelForProvider(providerID, fallback.model),
  variant: fallback.variant,
}

Affected Providers

Provider	Raw Model Name	Expected by API	Transform Applied
github-copilot	claude-sonnet-4-6	claude-sonnet-4.6	Hyphen → dot
github-copilot	gemini-3-pro	gemini-3-pro-preview	Add -preview suffix
google	gemini-3-pro	gemini-3-pro-preview	Add -preview suffix
google	gemini-3-flash	gemini-3-flash-preview	Add -preview suffix
anthropic	claude-opus-4-6	claude-opus-4-6	No transform needed

Changes

• src/hooks/model-fallback/hook.ts: Import transformModelForProvider and apply it in getNextFallback() return value
• src/hooks/model-fallback/hook.test.ts: Add tests verifying model name transformation for both github-copilot (claude-sonnet-4-6 → claude-sonnet-4.6) and google (gemini-3-pro → gemini-3-pro-preview) providers through the fallback chain

Scope

This fix targets only the hardcoded agent fallback chain (model-fallback hook). The runtime-fallback hook (auto-retry.ts) uses user-configured fallback_models where users specify provider-correct names directly, so no change is needed there.

Testing

• ✅ TypeScript typecheck passes (tsc --noEmit)
• ✅ All 5 model-fallback hook tests pass (including the new github-copilot and google transform tests)

[github-actions]

All contributors have signed the CLA. Thank you! ✅
_{Posted by the CLA Assistant Lite bot.}

[east-shine]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Surgical bug fix ensuring model ID format consistency across providers, with comprehensive test coverage for the transformation logic.}