auth/oidc: cache OIDC access token and refresh before expiry (#5175)

* auth/oidc: cache OIDC access token and refresh before expiry

- Use Config.TokenSource(ctx) once at init to create a persistent
  oauth2.TokenSource that caches the token and only refreshes on expiry
- Wrap with oauth2.ReuseTokenSourceWithExpiry for configurable early refresh
- Add tokenRefreshAdvanceDuration config option (default: 300s)
- Add unit test verifying token caching with mock HTTP server

* address review comments

* auth/oidc: fallback to per-request token fetch when expires_in is missing

When an OIDC provider omits the expires_in field, oauth2.ReuseTokenSource
treats the cached token as valid forever and never refreshes it. This causes
server-side OIDC verification to fail once the JWT's exp claim passes.

Add a nonCachingTokenSource fallback: after fetching the initial token, if
its Expiry is the zero value, swap the caching TokenSource for one that
fetches a fresh token on every request, preserving the old behavior for
providers that don't return expires_in.

* auth/oidc: fix gosec lint and add test for zero-expiry fallback

Suppress G101 false positive on test-only dummy token responses.
Add test to verify per-request token fetch when expires_in is missing.
Update caching test to account for eager initial token fetch.

* fix lint