fix(session): retry Codex server_is_overloaded stream errors

[ItsWendell]

Issue for this PR

Closes #25730

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Codex subscription streams can return transient overload errors as nested type: error events with error.code: server_is_overloaded and error.type: service_unavailable_error.

This PR routes that stream error through the existing retry path, and keeps the generic retry fallback aware of nested provider error codes/types.

How did you verify your code works?

• bun test test/session/retry.test.ts
• bun typecheck
• push hook: bun turbo typecheck

Screenshots / recordings

Not applicable, no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[spark4862]

Opened #30323 with overlapping overload handling plus stream_read_error, rate-limit stream chunks, and validation-wrapped envelopes. If maintainers prefer smaller PRs, I can split overload vs upstream retry paths.

[github-actions]

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