What

Summary

Phase 2B of issue #1074: ships mock_grpc_server_peer — a server-side gRPC framing peer composed on top of the existing mock_h2_server_peer's HTTP/2 SETTINGS exchange. With grpc_reply_mode::echo_unary, the peer reads one client request stream and replies with response HEADERS (:status: 200, content-type: application/grpc), one length-prefixed gRPC DATA frame, and trailing HEADERS (grpc-status: 0, END_STREAM).

Change Type

• Feature (new test infrastructure)
• Bugfix
• Refactor
• Documentation
• Test

Affected Components

• tests/support/mock_grpc_server_peer.{h,cpp} — new server-side gRPC peer
• tests/support/CMakeLists.txt — add new TU to network_test_support
• tests/support/README.md — document the new peer + composition example
• tests/unit/grpc_client_branch_test.cpp — one demo TEST_F covering the unary success path

Why

Problem Solved

grpc_client::call_raw post-connect success branches (http2_client::post wait, trailer-scan extracting grpc-status, grpc_message::parse on the response body) cannot be exercised by the Phase 2A mock_h2_server_peer because it never replies with HEADERS+DATA+trailers. This blocks branch-coverage expansion on protocols/grpc/client.cpp (#1063 follow-ups) and was explicitly called out in #1074's scope as Phase 2B.

Related Issues

• Part of infra(test): protocol-aware loopback peers + error injection (Phase 2 of #1060) #1074 (Phase 2 of infra: hermetic transport fixture for HTTP/2 / QUIC / WebSocket / gRPC unit tests #1060 — protocol-aware loopback peers)
• Part of Expand unit test coverage from 40% to 80% #953 (epic: 40% to 80% coverage)

Alternative Approaches Considered

• Reusing mock_h2_server_peer::echo_one: rejected because gRPC needs the trailer frame (grpc-status: 0) and content-type: application/grpc, neither of which are part of the bare h2 echo path.
• Pulling in a real gRPC server (grpc++): rejected as a hermeticity violation. Phase 2 specifically requires an in-process loopback peer with no external dependencies beyond the existing ASIO + OpenSSL stack.

Where

API Changes

None. New test-only infrastructure under tests/support/. No changes to src/ or public headers.

How

Implementation Highlights

1. Same SETTINGS handshake as mock_h2_server_peer: the new peer reuses the wire sequence (preface read -> server SETTINGS -> client SETTINGS read -> SETTINGS-ACK) so behavior diverges only after step 5.
2. HPACK encoding is dependency-free: :status: 200 uses static index 8 (one byte: 0x88); content-type: application/grpc and grpc-status: 0 use literal-without-indexing (0x00 prefix) with plain non-Huffman length-prefixed strings. All values fit in <=127 bytes so the simple 7-bit length form applies.
3. Trailer frame ordering: response HEADERS (END_HEADERS, no END_STREAM) -> DATA (no END_STREAM) -> trailing HEADERS (END_HEADERS + END_STREAM). The h2 client appends both header blocks into response_headers, so grpc_client sees :status and grpc-status in the same vector.
4. Hermetic discipline preserved: bound to 127.0.0.1:0 via the underlying tls_loopback_listener; RSA-2048 cert regenerated per construction; no DNS, no external network, no on-disk secrets.

Testing Done

Local C++ toolchain unavailable in the working environment — relying on CI for build / test verification. Code reviewed for HPACK correctness against src/protocols/http2/hpack.cpp (verified 0x88 indexed and 0x00 literal-without-indexing decode paths) and frame constructors against src/internal/protocols/http2/frame.h.

Test Plan for Reviewers

1. Build with the standard preset: cmake --preset release && cmake --build build
2. Run the new demo test: cd build && ctest -R CallRawSucceedsWithMockGrpcPeerEchoUnary --output-on-failure
3. Verify the test exercises:
   • grpc_client::call_raw returns a successful Result<grpc_message>
   • The response payload is the expected {'o', 'k'} body
   • peer.request_received() and peer.response_sent() both flip true
   • The captured peer.request_body() contains the gRPC 5-byte length prefix the client serialized

Breaking Changes

None.

Rollback Plan

Single commit — revert in one step. No migrations, no data, no public-API surface change.

Issue scope acknowledgement

This PR ships Phase 2B only. Phases 2C (mock_quic_peer_loop), 2D (network_test_friends.h friend-test injection), and 2E (frame_injector) remain. Per the #1074 acceptance criteria the issue stays open until all five phases land plus per-protocol demo tests and the README entries. This PR uses Part of, not Closes.

Part of #1074
Part of #953