Changes PR #2075 by ksylvan and mikaelpr: refactor: extract OAuth and auth logic from Codex client module

• Refactored the Codex client module by extracting all OAuth and authentication logic into a dedicated module, improving separation of concerns.
• Removed the OAuth flow, PKCE handling, and token refresh logic from codex.go, streamlining the client's core responsibilities.
• Removed the auth transport round-trip retry logic, simplifying the HTTP transport layer.
• Removed JWT parsing and token expiry utilities, along with unused OAuth types and helper structs, reducing dead code in the package.
• Added a test for SendStream HTTP error mapping and channel close behavior, improving test coverage for the client module.