[gabrielrinaldi]

Description

Preserve Gemini thoughtSignature values on ordinary text parts, including empty signature-only parts, through buffered decoding, stream assembly, and continuation encoding.

• store text signatures in existing thought_signature metadata
• preserve signed text boundaries in the Google response builder
• keep preserved content in response.message and response.context
• replay signatures through buffered and streaming request encoders
• apply the same behavior to Google Vertex Gemini
• keep unsigned text, thinking, and tool-signature behavior unchanged

Compatibility drift

This PR also resolves #1153.

• run broad replay selections only for active models that have fixture evidence
• run only scenarios that have a complete fixture contract
• report catalog models without fixtures as untested
• report retired models without running them
• keep exact model selections and recording behavior strict
• detect current reasoning capability metadata in usage tests
• align Google fixture requests with current provider requests, including thought signatures
• keep recorded provider responses unchanged

Current status

Rebased onto main through 9377b99. The current head is 514fac9.

All required GitHub checks pass, including the Elixir and OTP test matrix, formatting, Credo, Dialyzer, audit, docs, and package validation.

Testing

• mix mc 'google:*' --max-concurrency 4: 10 covered models pass, 29 models are untested, and 5 models are retired
• main test run: 5,427 tests passed, 11 skipped, and 240 excluded
• release build test: passed in an isolated rerun with the worktree dependency path
• mix quality: passed
• fixture validation: encoded requests match canonical JSON, and recorded provider responses are unchanged
• focused buffered and streaming tests cover signed text, empty signatures, mixed thinking and tool signatures, string-key metadata, continuation replay, and Google Vertex Gemini

Checklist

• Code follows the project style
• Documentation is updated
• Behavioral regression tests are included
• Compatibility drift from Reconcile Google model-compatibility fixtures and scenarios #1153 is resolved
• CHANGELOG.md is unchanged
• GitHub CI passes for the new commit

Related work

Closes #1153

• Downstream tracking: STR-47

[gabrielrinaldi]

The fix passes the default test suite and quality checks. I’ve left this as a draft because the Google compatibility matrix fails. The one fixture mismatch is confirmed on the upstream baseline.

Would you prefer the fixture issues handled separately, or resolved before marking this PR ready for review?