test(internal): expand coverage for http_error (#1035)

Add tests/unit/http_error_coverage_test.cpp exercising branches not
reached by tests/test_network_config_http_error.cpp:

- get_error_status_text default/unknown-code arm
- http_error::is_client_error / is_server_error boundary values
  (399, 400, 499, 500, 599, 600)
- parse_error::to_http_error with and without context
- http_error_response::make_error with/without detail and request_id
- build_json_error detail-fallback ternary, request_id if-branch,
  escape_json_string switch (", \\, \b, \f, \n, \r, \t, low
  control bytes, plain ASCII, high-bit bytes)
- build_html_error message / detail / request_id empty-vs-present
  branches and escape_html_string switch (& < > " ' default)
- deterministic ISO-8601 timestamp rendering through strftime path

Tests are hermetic (no network, filesystem, or sleeps) and hit every
switch arm of the two file-local escape helpers plus every if-branch
of the JSON and HTML builders.

Closes #1023

Author: kcenon

tests/CMakeLists.txt

@@ -4740,6 +4740,13 @@ add_network_test(network_http2_client_extended_coverage_test
 # conditions (Issue #1013)
 add_network_test(network_http_parser_coverage_test unit/http_parser_coverage_test.cpp)
 
+# HTTP error extended coverage: get_error_status_text default branch,
+# escape_json_string switch branches (all special chars, low control bytes,
+# plain ASCII, high-bit bytes), escape_html_string switch branches, and the
+# detail / request_id / message empty-vs-non-empty branches of
+# http_error_response::build_json_error and build_html_error (Issue #1023)
+add_network_test(network_http_error_coverage_test unit/http_error_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