feat(firestore): configure client to support 16MB documents

[bhshkh]

Configures the underlying gRPC client to explicitly support sending and receiving messages up to math.MaxInt32 bytes. This enables support for larger documents (up to 16MB) in the Go Firestore SDK.

Current Limits (Without this PR)

• Receive Limit: Already configured to math.MaxInt32 in the auto-generated apiv1 client.
• Send Limit: Not explicitly configured in the SDK. While the current grpc-go library implementation defaults to unlimited (math.MaxInt32) for client-side sending, explicitly setting it to math.MaxInt32 ensures robustness against future gRPC default changes and aligns with the expected behavior across other Firestore SDKs (which may have a 4MB default send limit).

Changes

This PR explicitly configures both MaxCallRecvMsgSize and MaxCallSendMsgSize to math.MaxInt32 in client.go to guarantee support for large payloads.

Also adds a regression test in integration_test.go:

• TestIntegration_VerifyGRPCLimits: Verifies the client can send and receive a 5MB payload (exceeding the default 4MB gRPC limits) using non-compressible random bytes. It asserts that both the write (Set) and read (Get) operations succeed.

Fixes : b/452640794

[gemini-code-assist]

Code Review

This pull request configures the Firestore client to support larger payloads by setting the gRPC maximum send and receive message sizes to math.MaxInt32 for both emulator and standard connections. It also introduces an integration test to verify these limits. Feedback on the changes suggests simplifying the test by removing the unnecessary error check for rand.Read, which is guaranteed to succeed.