Trim the salvaged commit to its two still-valid conversions:
- ChatCompletionsTransport.convert_messages (copy-on-write sanitize)
- QwenProfile.prepare_messages (copy-on-write normalize + cache_control)
Dropped from the original PR:
- agent/prompt_caching.py selective-copy: superseded by #57229 which
already rewrites apply_anthropic_cache_control on current main.
- Gemma extra_content narrowing (_model_consumes_thought_signature
'gemini or gemma' -> 'gemini' only) + its two tests: unrelated
behavior change reverting deliberate e8c3ac2f5; belongs in its own
PR with its own justification if pursued.
Conflict resolution: preserved main's newer timestamp-stripping
(#47868) inside the copy-on-write path.
63ddd022a2