test(http2): add client coverage test for #991 (#998)

Expand unit coverage of src/protocols/http2/http2_client.cpp via a new
tests/unit/http2_client_coverage_test.cpp with 39 focused tests:

- connect() guard clauses: empty host and empty host on default port
- connect() network error paths: unreachable port, invalid hostname,
  and TLS handshake failure against a plain-TCP listener
- disconnect() idempotency: no-op before connect, multiple calls,
  disconnect after a failed connect
- Destructor paths: fresh instance, after failed connect, after
  explicit disconnect
- set_settings() propagation: small/large/zero header_table_size,
  enable_push toggle, all fields changed at once
- set_timeout() edge values: very large, negative, persistence across
  failed connect
- Disconnected request/stream operations (expanded): get/post/put/del
  with headers and bodies, start_stream, write_stream with end_stream,
  close_stream_writer, cancel_stream, empty path
- http2_response: case-insensitive header lookup, duplicate-header
  order, empty-header lookup, binary body preservation, large body
- http2_stream: move semantics preserving buffers, default callbacks
  empty, move assignment preserving streaming flag
- stream_state: all pairs distinct

Tests avoid any reliance on a live HTTP/2 peer; a local plain-TCP
listener drives the TLS handshake failure branch.

Relates to #991

Author: kcenon

tests/CMakeLists.txt

@@ -4677,6 +4677,10 @@ add_network_test(network_http2_server_module_test unit/http2_server_test.cpp)
 # connection preface handling via raw TCP clients (Issue #992)
 add_network_test(network_http2_server_coverage_test unit/http2_server_coverage_test.cpp)
 
+# HTTP/2 client extended coverage: connect guard clauses, network error paths,
+# settings/timeout edge cases, disconnected request/stream operations (Issue #991)
+add_network_test(network_http2_client_coverage_test unit/http2_client_coverage_test.cpp)
+
 # Secure transport module tests
 add_network_test(network_secure_tcp_socket_module_test unit/secure_tcp_socket_test.cpp)
 # secure_messaging_udp_client/server excluded: depend on dtls_socket from