feat(firestore): configure gRPC message length limits for large documents

[ohmayr]

• Configure grpc.max_send_message_length and grpc.max_receive_message_length options on production and emulator gRPC channels.
• Update unit tests in test_base_client.py to assert gRPC message size options.
• Add system tests for large document CRUD and pipeline execution across sync and async clients.

[gemini-code-assist]

Code Review

This pull request configures unlimited gRPC send and receive message sizes for Firestore clients, including emulator channels, to support handling large documents. It also adds system tests for both synchronous and asynchronous operations with large documents on Enterprise DB, along with corresponding unit test updates. The review feedback highlights a critical issue in the new asynchronous system tests: passing the asynchronous doc_ref.delete coroutine to a synchronous cleanup fixture will cause cleanup to fail and raise a RuntimeWarning. It is recommended to use try...finally blocks to explicitly await the deletion of the documents.

[parthea]

LGTM, but holding off on formal approval. Please could you address the feedback from gemini-code-assist?