Port gRPC CallCredentials contracts

Continue Bun corpus parity at position 3970 by porting js/third_party/grpc-js/test-call-credentials.test.ts without modifying the copied corpus.

Acceptance criteria:

• Implement CallCredentials.createFromMetadataGenerator and immutable, chainable composition.
• Generate metadata asynchronously with deterministic composition order and merge repeated values correctly.
• Reject generator failures as diagnosable Error objects with a useful CallCredentials stack frame.
• Pin the copied file in the permanent third-party mini-suite with zero failures, todos, or unsupported cases.
• Push the completed conventional commit to main as Chris Breuer without coauthors and update #66.

Completed in 30bb1f0 (pushed to main).

• Ported all 6 CallCredentials tests with zero todos, failures, or unsupported cases.
• Added validated metadata-generator construction, immutable chainable composition, deterministic async metadata merging, and reusable Metadata.merge.
• Generator failures remain Error objects and receive an explicit CallCredentials.generateMetadata stack frame identifying the failing generator.
• Pinned js/third_party/grpc-js/test-call-credentials.test.ts in the permanent mini-suite; focused and permanent gates pass.