[codex] add generic OAuth2 outgoing auth

[winhowes]

Summary

• Add a generic outbound oauth2 auth plugin for client_credentials and refresh_token token exchanges.
• Cache access tokens until one minute before expiry and reuse rotated refresh tokens in memory.
• Allow plugin-declared secret refs so client_secret and refresh_token references are validated like existing secrets fields.
• Document the new plugin configuration.

Validation

• go test ./app/auth/plugins/oauth2 -coverprofile=/tmp/oauth2.cover reports 100.0% statement coverage for the new OAuth2 plugin package.
• go test ./... passes.

[winhowes]

@codex review

[chatgpt-codex-connector]

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Reviewed commit: bac7f884b2

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[winhowes]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 3adafa6b70

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[winhowes]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 5c08c612d4

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[winhowes]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".