Port gRPC channel credential contracts

Tracks copied Bun corpus position 3973 from #66: js/third_party/grpc-js/test-channel-credentials.test.ts.

Acceptance criteria:

• lower the file's mixed value/type imports and credential mock TypeScript syntax without weakening unsupported-syntax detection
• implement insecure and SSL channel credentials with private-key/certificate pair validation
• support call-credential composition and expose connection/call credential state through the gRPC-compatible APIs
• propagate generated metadata through unary client/server calls and client metadata events
• preserve causes and add channel/call operation stack context on asynchronous failures
• pass all nine active upstream checks, retain the one explicit upstream todo, and pin the file in the permanent third-party suite

Parent: #66

Completed in 259f14fee42e992b57d222544941dde8542fe7a7 and pushed to main.

• Added secure/insecure ChannelCredentials, SSL key/certificate pair validation, call-credential composition, connection option inspection, and gRPC build-subpath exports.
• Added Metadata.set, generated-metadata merging, server-call metadata delivery, and client metadata events for unary calls.
• Added cause-preserving ChannelCredentials, ServerCredentials, EchoService, and metadata-generation operation stacks; callback exceptions are rethrown instead of being misreported as transport failures.
• Focused result: 9 passed, 1 explicit upstream todo, 0 failed, 0 unsupported.
• Credential diagnostic regression, SQL contracts, and the complete permanent third-party suite pass.