fix(google): keep thought parts out of visible text

[Yuxin-Qiao]

Summary

• Keep Google response parts marked thought: true out of ordinary assistant text in both SSE and buffered JSON parsing.
• Route provider-marked thought text through reasoning_raw_delta while preserving ordinary text, function-call ordering, inline images, and Antigravity/Vertex thought-signature observation.
• Share one classifier across both response modes and document the visibility boundary and decision rationale.
• This PR is the current-dev rebase of fix(google): keep thought parts out of visible text #1508 and supersedes that draft.
• Closes [Bug] google: do not emit thought:true part text as visible assistant text #1503

Verification

• bun run typecheck — passed.
• bun test tests/google-hardening.test.ts tests/google-antigravity-wire.test.ts tests/google-vertex-thought-signature.test.ts — 81 passed, 0 failed.
• bun run privacy:scan — passed.
• Base: latest origin/dev at the time of push.
• No GUI change.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.
• Local focused verification is green.
• Branch is based on the latest dev commit.
• Independent review completed.
• Ready-for-review confirmation is present.

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Review readiness checklist

• ⬜ All CI tests are green on my local testing.
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• ⬜ I resolved all correct Codex and CodeRabbit findings.
• ⬜ My PR is ready for review.

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[Ingwannu]

Thanks for carrying this forward. I compared the patch against #1508: the Google adapter, regression tests, and structure note are the same 3-file 106 insertions / 5 deletions implementation, while #1508 has now been rebased further onto current dev@fd59bd270. Keeping two review heads for the same fix would only split CI and review state, so I am closing this as a duplicate of #1508. Please continue any review on #1508.