🐛 fix(rmcp-client): refresh OAuth tokens using expires_at

[LaelLuo]

Summary

• persist OAuth credential expiry timestamps and rehydrate expires_in
• proactively refresh rmcp OAuth tokens when expires_at is near, then persist

Testing

• just fmt
• just fix -p codex-rmcp-client
• cargo test -p codex-rmcp-client

Fixes #6572

[chatgpt-codex-connector]

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[LaelLuo]

Updated persist_if_needed so that we reuse the previously stored expires_at whenever the token response is unchanged and only compute a new timestamp when rmcp returns fresh credentials. That keeps the expiry anchored to the original issuance time and allows the refresh skew logic to trigger instead of drifting indefinitely.

[LaelLuo]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

ℹ️ About Codex in GitHub

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• Mark a draft as ready
• Comment "@codex review".

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