What

Adds 12 new TEST_F cases under the existing GrpcClientHermeticTransportTest fixture in tests/unit/grpc_client_branch_test.cpp (+317 LOC, no new test files) that compose the Phase 2A mock_h2_server_peer shipped in PR #1075 to bring grpc_client into a fully SETTINGS-exchanged state, then exercise public methods that previously early-returned via is_connected() in the existing disconnected-state suites.

Newly reached paths in src/protocols/grpc/client.cpp

• is_connected() compound condition after the inner http2 SETTINGS exchange completes (lines 1120-1122)
• wait_for_connected() polling loop hitting the success branch (lines 1125-1139)
• second connect() short-circuit through the already_connected branch (lines 1035-1042)
• target() getter post-handshake (lines 1141-1144)
• call_raw() connected path: header build, grpc-timeout header formatting, metadata copy loop, http2_client::post forwarding, future-timeout error branch (lines 1146-1245)
• call_raw() post-connect deadline-exceeded branch (lines 1188-1202)
• call_raw_async() connected submit_task path with callback delivery (lines 1340-1350)
• server_stream_raw() / client_stream_raw() / bidi_stream_raw() connected paths: header build, start_stream forwarding, shared_*_holder allocation, ok() unique_ptr return (lines 1352-1611)
• disconnect() second-call idempotent path after a real handshake (lines 1107-1118)

Why

Part of #1063 and Part of #953 (epic: 40% → 80% coverage).

grpc_client.cpp line coverage stood at 22.6% line / 9.5% branch as of the 2026-04-26 lcov run (workflow 24947193873). PR #1069 added 36 disconnected-state TEST_F / TEST cases, but the remaining gap was concentrated in the post-handshake code path that needed an in-process HTTP/2 peer to drive is_connected() == true in CI.

Because grpc_client::impl instantiates an internal http2::http2_client and delegates connect() to its connect(host, port), the Phase 2A mock_h2_server_peer shipped in PR #1075 is sufficient to push grpc_client past the connection gate without requiring a separate mock_grpc_server_peer to be built first. This PR consumes that infrastructure to actually expand grpc_client.cpp coverage.

The acceptance criteria of >= 80% line / >= 70% branch is not reached by this PR alone — it cannot be, until Phase 2A.2 of #1074 lands mock_h2_server_peer HEADERS+DATA reply support. Without server replies, the server_stream_reader_impl::on_data / on_headers / on_complete callback wiring, the client_stream_writer_impl round-trip, the bidi_stream_impl read/write/finish round-trip, the call_raw happy-path completion (HTTP/2 200 → trailer parse → status code extraction → ok(grpc_message)), and the official-grpcpp wrapper paths all remain unreachable from a hermetic test. This PR therefore uses Part of #1063, not Closes.

Where

No changes under src/, no new test files, no CMake change (the file is already at tests/CMakeLists.txt:4916-4918 linked to network::test_support, which already statically links mock_h2_server_peer.cpp from PR #1075).

How

Each new TEST_F:

1. Constructs a mock_h2_server_peer peer(io()) on the fixture's shared io_context.
2. Calls make_connected_grpc_client(peer, default_timeout) which spawns a worker thread that runs client->connect() synchronously against 127.0.0.1:peer.port().
3. Waits on wait_for([&]() { return peer.settings_exchanged(); }, std::chrono::seconds(3)) — by which time the peer has read the preface, sent server SETTINGS, read client SETTINGS, and sent SETTINGS-ACK; the inner http2_client has flipped is_connected_ to true and grpc_client::is_connected() returns true.
4. Drives the targeted post-connect public method, asserting the expected Result<T> outcome.
5. Calls disconnect() which tears down the inner http2 client (emitting GOAWAY, drained by the peer), then joins the connector thread.

Tests reaching the call_raw post-timeout branch use a 150 ms default_timeout to bound execution time. Streaming methods use the default 2 s timeout for the connect path; their post-connect calls return immediately since they only forward to http2_client::start_stream which allocates a stream id without waiting on a future.

Coverage Scope (Honest Assessment)

Local C++ toolchain (cmake/g++/ninja) is not available in this dev environment; the coverage.yml workflow is the authoritative verification surface (PR #1071 / #1075 / #1076 precedent). Expected outcome:

• grpc_client.cpp line coverage strictly increases above the 22.6% baseline.
• HEADERS+DATA reply paths (callbacks, trailer parsing, happy-path completion) remain uncovered — Phase 2A.2 of infra(test): protocol-aware loopback peers + error injection (Phase 2 of #1060) #1074 will unblock.
• Issue test(grpc): expand client.cpp coverage to 80% line / 70% branch #1063 stays open (Part of, not Closes); the issue's >= 80% line / >= 70% branch acceptance criteria becomes reachable only after Phase 2A.2 lands and a final follow-up test PR drives the success paths.

Part of #1063
Part of #953