test: add unit tests for OAuth2CredentialRefresher

[sarathfrancis90]

Link to Issue or Description of Change

2. Or, if no issue exists, describe the change:

Problem:
core/src/auth/oauth2/oauth2_credential_refresher.ts (OAuth2CredentialRefresher) had no unit tests and 0% coverage, while its sibling OAuth2CredentialExchanger is already covered by core/test/auth/oauth2/oauth2_credential_exchanger_test.ts. adk-python also has an equivalent test for its refresher, so the JS side was missing parity.

Solution:
Add core/test/auth/oauth2/oauth2_credential_refresher_test.ts, following the established pattern used by the exchanger test (mocking oauth2_utils.js). The suite covers the real branches of the class:

• isRefreshNeeded: no oauth2 field, oauth2 without expiresAt, token not expired, and token expired.
• refresh: missing oauth2, missing auth scheme, missing refresh token, refresh not needed, missing token endpoint, missing clientId/clientSecret, successful refresh, response-field fallback (preserving existing values when the response omits them), refresh-token request body construction, and fetch failure (returns the original credential).

This brings OAuth2CredentialRefresher to coverage parity with OAuth2CredentialExchanger and mirrors the equivalent test in adk-python. No source code changes are included; this is test-only.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Local results:

• npm test: 1869 passed, 28 skipped (171 test files). The new file contributes 14 passing tests.
• npm run test:coverage: core/src/auth/oauth2/oauth2_credential_refresher.ts goes from 0% to 100% (statements 64/64, branches 23/23, functions 2/2).
• npm run lint: clean.
• npm run format:check: clean.
• npm run docs:check: clean.

Manual End-to-End (E2E) Tests:

Not applicable. This is a test-only change with no runtime behavior changes.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Test-only change. No prior issue is required for a test addition. The new file includes the Apache 2.0 license header matching the other test files in the repository.