fix(config): resolve Antigravity/Native Gemini config conflict

[Sladck]

This PR fixes a conflict where enabling Antigravity auth would overwrite existing native Gemini CLI configurations.

Changes:

1. Deep Merge for Google Provider: Modified add-provider-config.ts to merge Antigravity settings with existing Google provider configuration instead of replacing it. This preserves API keys and other custom settings.
2. Model ID Prefix Injection: Updated provider-model-id-transform.ts and fallback-chain-resolution.ts to automatically inject the antigravity- prefix into model IDs when Antigravity integration is active. This ensures the correct model IDs (e.g., antigravity-gemini-3-pro) are used in the generated agent configuration while still allowing fallback to native IDs if needed.

Why:
Users utilizing both Antigravity auth (for high-tier tasks) and native Gemini CLI auth (for other tasks or fallbacks) were experiencing configuration overrides and incorrect model ID usage.

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Summary by cubic

Fixes the conflict between Antigravity auth and native Gemini config. Preserves existing Google provider settings and uses Antigravity-prefixed model IDs only when Antigravity is active.

• Bug Fixes
  • Deep-merge Antigravity Google provider config with existing values to keep API keys and custom settings.
  • Auto-prefix Gemini model IDs with antigravity- for Google when Antigravity is enabled; keep native IDs otherwise.
  • Align fallback chain resolution with the same model ID logic to ensure valid provider/model pairs.

^{Written for commit 8263dda. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

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

_{Auto-approved: PR adds provider model transformations safely; no regressions detected by AI and changes are minor and well-contained.}