feat(firestore): Add support for 16MB documents

[dlarocque]

• Adds support for 16 MB documents in Firestore by increasing the gRPC message size limit to 17MB.
• Increases gRPC flow control window default from 64 KB to 256 KB, to reduce latency for large document reads.

If a regression is introduced by increasing the flow control window, this can be overwritten:

const firestore = new Firestore.Firestore({ 
  // ...
  grpcOptions: { 'grpc-node.flow_control_window': 64 * 1024 }
});

[gemini-code-assist]

Code Review

This pull request updates the Firestore client to support larger documents by increasing the gRPC max receive and send message lengths to 17MB and configuring flow control window sizes. It also adds a new integration test suite (large_document.ts) to verify handling of large documents (up to 15.9MB) and rejection of oversized payloads. The review feedback correctly identifies an issue in the oversized payload test where a successful write would throw a generic error that gets caught and fails on an unrelated assertion, masking the true failure; a cleaner assertion pattern is suggested.