Reconcile Google model-compatibility fixtures and scenarios #1153

Summary

The Google model-compatibility replay data is not aligned with the current provider request format and model catalog.

The last recorded run from PR #1127 selected 44 models. It reported 39 active failures and 5 retired models. The failures included missing fixtures, recorded-request mismatches, and unsupported options for newer models.

One confirmed baseline mismatch is:

• fixture: google/gemini_2_5_flash/usage.json
• field: canonical_json.generationConfig.maxOutputTokens
• recorded value: 500
• current value: 10

The same mismatch was reproduced on the upstream Google provider before the text thought-signature change in PR #1127. This compatibility work is therefore separate from that fix.

Work

• run the complete Google compatibility matrix against current main
• classify missing fixtures, request drift, retired models, and unsupported scenarios
• update fixtures or scenario expectations where current behavior is correct
• add explicit exclusions only when the model or scenario is not supported
• make the active Google compatibility matrix pass, or document each remaining expected exclusion

Reproduction

mix mc 'google:*' --max-concurrency 4
mix mc google:gemini-2.5-flash --scenario usage --debug --max-concurrency 1

Related: #1127