Prefer the model list fetched from the backend for SIWC users

[jeevnayak]

Summary

• For SIWC users, update the model list merging logic to prefer the model list fetched from the backend over the bundled model list (this is needed for special cases where users have a more limited set of models they're allowed to use)
• Add or update tests covering the revised cache behavior

Testing

• Added/updated unit tests in codex-rs/models-manager/src/manager_tests.rs
• Not run (not requested)

[github-actions]

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[sayan-oai]

looks good!

please add an integration test for the e2e (subset/single model returned by /models using ChatGPT auth, model listing flow only has those models and no bundled models).

we prefer integration tests because codex changes unit tests too freely :)

[jeevnayak]

I have read the CLA Document and I hereby sign the CLA