test(openai): enforce ownership across Responses lifecycle

[leseb]

Summary

• capture the immutable state owner before inference or body-first processing for every Responses request that will persist state
• carry the captured owner through buffered and streaming persistence instead of re-reading mutable request context at response time
• extend lifecycle tests for owner-scoped retrieval, input items, continuation, compaction, deletion, and pending MCP approval consumption
• preserve indistinguishable not-found behavior for missing and unauthorized state

Related issue

Closes #1166

Validation

• Unit tests: cargo test -p praxis-ai-apis openai::responses (2036 passed)
• Integration tests: cargo test -p praxis-tests-integration --test suite -- examples::openai_response_store (7 passed, 2 container-backed tests ignored locally)
• make lint

Checklist

• I reviewed every changed line and can explain the change.
• New capabilities include an example config and functional example test. Existing response-store examples exercise this lifecycle boundary; end-to-end cross-owner validation is in test(openai): validate persisted state ownership end to end #1198.
• User-facing behavior and generated documentation are updated. Existing ownership documentation remains current.
• Performance-sensitive changes include appropriate benchmark or load-test evidence. Not applicable: the request captures one existing owner value and moves it through request state.
• Commits are signed and include a Signed-off-by trailer.

Breaking changes

None.