Support flow-controlled gRPC client requests

[wasphin]

What problem does this PR solve?

Issue Number: #1346, #1770, #1983, #2117

Problem Summary:

The gRPC client previously attempted to serialize the entire request into
HTTP/2 DATA frames immediately.

When the request size exceeded the peer's available connection-level or
stream-level flow-control window, frame generation failed and the RPC returned
an error instead of waiting for WINDOW_UPDATE.

The client needs to retain unsent DATA while waiting for the peer to restore
the flow-control window. This retained DATA must be bounded and released with
the RPC lifecycle to prevent excessive memory usage when the peer does not send
WINDOW_UPDATE.

What is changed and the side effects?

Changed:

• Split client request DATA frames according to both connection-level and
  stream-level HTTP/2 flow-control windows.

• Store DATA that cannot be sent immediately in H2StreamContext.

• Resume sending pending DATA when a valid WINDOW_UPDATE restores the
  available window.

• Track the total pending request DATA size for each H2 connection.

• Use socket_max_unwritten_bytes as the upper bound for pending request DATA.

• Reject new requests with EOVERCROWDED after a connection reaches the
  pending DATA limit.

Side effects:

• Performance effects:
  • Requests blocked by remote flow control remain buffered until the peer
    sends WINDOW_UPDATE, the RPC finishes, or the stream is removed.
  • Pending DATA accounting and flow-control operations use the existing H2
    stream mutex.
  • Fragmented requests may require additional DATA frame serialization and
    socket writes.
• Breaking backward compatibility: N/A

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull request overview

Adds client-side HTTP/2 (gRPC) request flow-control compliance in bRPC by buffering unsent request DATA when remote connection/stream windows are exhausted, then resuming transmission on WINDOW_UPDATE, with a per-connection pending-data cap to bound memory usage.

Changes:

• Buffer unsent HTTP/2 request DATA per stream and flush it when WINDOW_UPDATE/SETTINGS increase the usable window.
• Track per-connection total pending request DATA and reject new requests with EOVERCROWDED once the configured cap is reached.
• Extend/adjust unit tests to validate request buffering, DATA splitting, and window-update behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/brpc_http_rpc_protocol_unittest.cpp	Updates HTTP/2 flow-control test to validate buffering/cleanup behavior.
test/brpc_h2_unsent_message_unittest.cpp	Adds new unit tests for DATA splitting, buffering, and pending-data limit behavior.
test/brpc_grpc_protocol_unittest.cpp	Adds regression test for sending large gRPC requests with small remote flow-control windows.
src/brpc/policy/http2_rpc_protocol.h	Introduces per-stream pending request DATA and per-connection pending-data accounting APIs.
src/brpc/policy/http2_rpc_protocol.cpp	Implements buffering/flush logic, SETTINGS/WINDOW_UPDATE integration, and pending-data limit checks.

Suppressed comments (2)

test/brpc_h2_unsent_message_unittest.cpp:59

• PopFrame() passes &(*payload)[0] to cutn() even when frame.payload_size == 0. Taking operator[] on an empty std::string is undefined behavior and can trip sanitizers if a 0-length frame is ever parsed in this test.

    payload->resize(frame.payload_size);
    CHECK_EQ(frame.payload_size,
             buf->cutn(&(*payload)[0], frame.payload_size));
    return frame;

test/brpc_grpc_protocol_unittest.cpp:298

• After switching to an ephemeral port, initialize the client channel using server.listen_address() to avoid hard-coding the address/port.

    brpc::Channel channel;
    brpc::ChannelOptions channel_options;
    channel_options.protocol = g_protocol;
    channel_options.timeout_ms = 10000;
    ASSERT_EQ(0, channel.Init("127.0.0.1:8012", "", &channel_options));

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