Restore persisted model provider on thread resume

[etraut-openai]

Fixes #15219.

Why

thread/resume should continue a persisted thread with the same model provider that created the thread. The app server already restores the persisted model and reasoning effort before resuming, but it was leaving model_provider unset. If a user created a thread with one provider and later switched their active profile to another provider, resumed encrypted history could be sent to the wrong endpoint and fail with invalid_encrypted_content.

The thread metadata already records the original provider, so resume should apply it when the caller has not explicitly requested a different model/provider/reasoning configuration.

What changed

This updates merge_persisted_resume_metadata in app-server/src/codex_message_processor.rs to copy ThreadMetadata::model_provider into ConfigOverrides::model_provider alongside the persisted model.

The existing resume metadata tests now also assert that:

• the persisted provider is restored for normal resume
• explicit model, provider, or reasoning-effort overrides still prevent persisted resume metadata from being applied
• a thread with no persisted model or reasoning effort still resumes with its persisted provider

Verification

• cargo test -p codex-app-server passed the app-server unit tests, including the updated resume metadata coverage. The broader integration portion of that command failed in an unrelated environment-sensitive skills-budget warning assertion, where this run saw 8 omitted skills instead of the expected 7.
• just fix -p codex-app-server completed successfully.