Issue #1119 (Round 3 of #953): expand http2_client.cpp coverage to
>=70% line / >=50% branch.

Round 6 (#1115 / PR f3c289f) and Round 2 (#1117 / PR 7af3bc5) raised
http2_client.cpp coverage by direct-dispatching server-originated frames
through http2_client_test_access::process_frame. Both rounds targeted
handlers whose entry arms are reachable without an entry in streams_;
anything that runs after the stream lookup succeeds remained
unmeasured.

This commit extends the friend access class with stream-seeding helpers
(seed_stream, set_stream_callbacks, stream_state_of, stream_window_size_of,
append_response_header, response_body_of, build_headers,
allocate_stream_id, next_stream_id) and adds 38 TEST cases driving the
known-stream success-path branches:

- handle_rst_stream_frame: close-and-fulfill arm with both CANCEL and
  REFUSED_STREAM error codes
- handle_goaway_frame: populated streams_ loop body covering streams
  above last_stream_id (closed and promise.set_value), at-or-below
  (kept open), and already-closed (skipped)
- handle_window_update_frame: known-stream non-zero stream_id branch
  including INT32_MAX boundary
- handle_data_frame: non-streaming buffer-append, end_stream success
  with status-code extraction (200), malformed-status std::stoi catch
  arm, streaming on_data fan-out, streaming end_stream on_complete,
  per-stream WINDOW_UPDATE branch, connection-level WINDOW_UPDATE
  branch, max-uint31 stream id, and 16384-byte payload round-trip
- handle_headers_frame: accumulate (end_headers without end_stream),
  end_stream success with HPACK static-index decode for status 200 and
  404, malformed-status catch arm, streaming on_headers, streaming
  end_stream on_complete with status 500
- process_frame: dynamic_cast-fails / break / fall-through arm for all
  seven dispatched frame types, plus default arm for priority,
  push_promise, and continuation
- Private helpers: build_headers empty-path-to-/ substitution,
  pseudo-header sequencing, additional-header pseudo-header skip;
  allocate_stream_id RFC 7540 5.1.1 odd-id contract

Production header compiles byte-identical when BUILD_TESTS=OFF; all
friend declarations remain inside the existing
NETWORK_ENABLE_TEST_INJECTION gate. No new friend declarations needed
because the new accessors are member functions of the existing
http2_client_test_access class.

Local verification: 38/38 PASSED in 1 ms wall-time on Apple Clang
Debug build; existing network_http2_client_branch_test non-network
suites (34 tests across 8 suites) continue to pass.

Part of #953 coverage expansion.