Fix the token expiration logic in SR Oauth

[fangnx]

What

Checklist

• Contains customer facing changes? Including API/behavior changes
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

References

JIRA:

Test & Review

Open questions / Follow-ups

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[Copilot]

Pull request overview

This PR fixes a critical bug in the token expiration logic for Schema Registry OAuth authentication. The previous implementation incorrectly calculated when tokens should be refreshed, causing them to be refreshed much earlier than intended.

Changes:

• Fixed the token expiration calculation to properly compute the refresh buffer as expires_in * (1 - token_expiry_threshold) instead of expires_in * token_expiry_threshold
• Applied the fix consistently to both sync and async implementations
• Improved variable naming from expiry_window to refresh_buffer for better clarity

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/confluent_kafka/schema_registry/_sync/schema_registry_client.py	Fixed token expiration logic in the synchronous OAuth client to correctly calculate when tokens need refreshing
src/confluent_kafka/schema_registry/_async/schema_registry_client.py	Fixed token expiration logic in the asynchronous OAuth client to correctly calculate when tokens need refreshing

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[Copilot]

The existing test case test_expiry() in tests/schema_registry/_sync/test_bearer_field_provider.py (and the async equivalent) will fail with this fix because it uses inconsistent test data. The test sets expires_at = time.time() + 2 and expires_in = 1, which doesn't match real OAuth token behavior where expires_at should be approximately current_time + expires_in. The test needs to be updated to use consistent values, for example: {'expires_at': time.time() + 1, 'expires_in': 1} or adjust the sleep duration and assertions accordingly.

[Copilot]

The existing test case test_expiry() in tests/schema_registry/_async/test_bearer_field_provider.py will fail with this fix because it uses inconsistent test data. The test sets expires_at = time.time() + 2 and expires_in = 1, which doesn't match real OAuth token behavior where expires_at should be approximately current_time + expires_in. The test needs to be updated to use consistent values, for example: {'expires_at': time.time() + 1, 'expires_in': 1} or adjust the sleep duration and assertions accordingly.