test: add unit tests for OAuth2CredentialRefresher

[nuthalapativarun]

Problem

OAuth2CredentialRefresher in core/src/auth/oauth2/oauth2_credential_refresher.ts has no unit test coverage.

Solution

Add unit tests covering isRefreshNeeded and refresh methods, including happy paths, no-op conditions (missing oauth2/refreshToken fields), and error handling.

Testing

• All new tests pass: npm test
• No existing tests broken

Checklist

• I have performed a self-review of my code
• Tests added and passing
• Manual E2E verified (N/A — test-only change)

[nuthalapativarun]

Superseded by #464 which combines all three test files into a single PR.