What

Adds 12 new TEST_F cases under the existing Http2ClientHermeticTransportTest fixture in tests/unit/http2_client_branch_test.cpp (+367 LOC, no new test files) that compose the Phase 2A mock_h2_server_peer shipped in PR #1075 to bring http2_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/http2/http2_client.cpp

• connect() second-call short-circuit through the already_exists branch (lines 81-88)
• send_request() / get() / post() create_stream + build_headers + encoder_.encode + send_frame on the connected path, terminating in the future.wait_for(timeout_) branch (lines 723-844, especially 817-826)
• start_stream() / write_stream() / cancel_stream() / close_stream_writer() connected paths plus their not_found error branches with an unallocated stream id (lines 306-462)
• close_stream_writer() idempotent early-return on a half_closed_local stream (lines 412-415)
• set_settings() post-handshake call site updating HPACK encoder / decoder table sizes (lines 299-304)
• disconnect() GOAWAY emit + stop_io path followed by the second-call early-return on a torn-down handshaken client (lines 210-237)

Why

Part of #1062 and Part of #953 (epic: 40% → 80% coverage).

http2_client.cpp line coverage stood at 18.8% line / 9.9% branch as of the 2026-04-26 lcov run (workflow 24947193873). PR #1068 added 30 disconnected-state TEST_F cases but the remaining gap was concentrated in the post-handshake code that needed an in-process peer to drive is_connected() == true in CI. PR #1075 shipped that peer (mock_h2_server_peer — connection preface read, server SETTINGS send, client SETTINGS read, SETTINGS-ACK send) and added one demo TEST_F. This PR is the first follow-up that uses the peer to actually expand http2_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, all handle_headers_frame / handle_data_frame / handle_rst_stream_frame / handle_goaway_frame / handle_window_update_frame / handle_ping_frame paths remain unreachable from a hermetic test, and the happy-path completion of send_request() (status code extraction, promise.set_value(...)) cannot be driven. This PR therefore uses Part of #1062, not Closes.

Where

No changes under src/, no new test files, no CMake change (the file is already in add_network_test(http2_client_branch_test ...) and network::test_support is already linked 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_client(peer, client_id) which spawns a worker thread that runs client->connect("127.0.0.1", peer.port()) synchronously.
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; client's is_connected_ is therefore true.
4. Drives the targeted post-connect public method, asserting the expected Result<T> outcome.
5. Calls disconnect() which emits GOAWAY (drained by peer) and tears down state, then joins the connector thread.

Tests reaching the send_request() timeout branch use a 150 ms set_timeout(...) to bound execution time. Other tests use the default 2 s timeout for the connect path; their post-connect calls return immediately since they don't wait 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 precedent). Expected outcome:

• http2_client.cpp line coverage strictly increases above the 18.8% baseline.
• HEADERS+DATA reply paths remain uncovered (Phase 2A.2 of infra(test): protocol-aware loopback peers + error injection (Phase 2 of #1060) #1074 will unblock).
• Issue test(http2): expand http2_client.cpp coverage to 80% line / 70% branch #1062 stays open (Part of, not Closes); the issue's >= 80% line / >= 70% branch acceptance criteria becomes reachable only after Phase 2A.2 lands.

Part of #1062
Part of #953