feat(core): implement silent fallback for Plan Mode model routing

[jerop]

Summary

Implement silent fallback for Plan Mode to ensure an uninterrupted user experience when Pro models are unavailable or inaccessible.

Details

This PR introduces a "Unified Injection" approach to enforce silent fallback when the system is in Plan Mode. By intercepting the ModelPolicyChain resolution in resolvePolicyChain, all fallback actions are overridden to 'silent' if ApprovalMode.PLAN is active. This ensures that failures in the primary planning model (Pro) automatically trigger a transition to the next available model (Flash) without interrupting the user with prompts or getting "stuck".

• Exported SILENT_ACTIONS from policyCatalog.ts.
• Refactored resolvePolicyChain in policyHelpers.ts to apply silent overrides.
• Added unit tests in policyHelpers.test.ts verifying silent fallback for both legacy and dynamic routing paths.
• Updated docs/cli/plan-mode.md to document the new behavior.

Related Issues

Closes #25110

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request implements a silent fallback mechanism for Plan Mode to ensure a seamless user experience. By overriding model policy actions to 'silent' when Plan Mode is active, the system can automatically transition to alternative models without interrupting the user with prompts or blocking execution when primary models are unavailable.

Highlights

• Silent Fallback Implementation: Introduced a unified injection mechanism in resolvePolicyChain that forces silent fallback actions when the system is in Plan Mode.
• Policy Configuration: Exported SILENT_ACTIONS from policyCatalog.ts to allow consistent application of silent fallback policies across the core module.
• Testing and Documentation: Added unit tests to verify silent fallback behavior for both legacy and dynamic routing paths, and updated the CLI documentation to reflect this change.

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[github-actions]

Size Change: +242 B (0%)

Total Size: 34.1 MB

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[gemini-code-assist]

Code Review

This pull request updates the documentation to reflect automatic model fallback behavior and implements a unified silent policy injection when the CLI is in Plan Mode. The resolvePolicyChain function was refactored to ensure consistent application of dynamic slicing and to override policy actions with SILENT_ACTIONS during planning. A review comment identifies a potential issue where SILENT_ACTIONS is assigned by reference, which could lead to shared state across policies; it is recommended to use the spread operator to provide each policy with its own instance of the actions map.

[jerop]

/patch stable

[github-actions]

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• Channels: stable
• Commit: 050c30330e2bb71558f51b2a4482d8970a7a84cb
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[github-actions]

🚀 [Step 2/4] Patch PR Created!

📋 Patch Details:

• Environment: prod
• Channel: stable → will publish to npm tag latest
• Commit: 050c30330e2bb71558f51b2a4482d8970a7a84cb
• Hotfix Branch: hotfix/v0.38.0/0.38.1/stable/cherry-pick-050c303/pr-25317
• Hotfix PR: #25466

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• Environment: prod
• Channel: stable → publishing to npm tag latest
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✅ [Step 4/4] Patch Release Complete!

📦 Release Details:

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