ci: build the HTTP server test hooks on the Linux debug leg

[EdmondDantes]

The Linux debug leg configures with --enable-http-server-test-hooks, so the 20 phpt gated on the _http_* functions run in CI: the log sink tests in core/ (019, 020, 022, 024, 026, 030, 031, 056), reactor_pool/001 to 010, and telemetry/001 to 003. Nothing ran them before.

Merge after true-async/php-async#311. Built with the flag, core/030 crashed with SIGSEGV on every run (3 of 3 under run-tests): php-async's io_pipe_write_cb cleared io on a write request the log sink had already freed inside the completion notify, and the next write took its address (true-async/php-async#310; asynctest case true-async/php-src#39). With #311, the 20, the four tests behind --enable-tas-test-hooks and the rest of reactor_pool/ pass 35 of 35, 3 runs of 3.

Also closes plan item 16 (#348, done in PR 349) in dev/PLAN.md.

[github-actions]

Coverage

Total lines: 83.95% → 84.00% (+0.05 pp)

File	Baseline	Current	Δ	Touched
src/core/http_connection.c	83.59%	83.67%	+0.08 pp
src/formats/multipart_processor.c	88.06%	88.48%	+0.42 pp
src/http3/http3_listener.c	77.66%	79.01%	+1.35 pp
src/http3/http3_packet.c	85.22%	90.43%	+5.22 pp
src/http_server_class.c	76.12%	76.05%	-0.07 pp
src/websocket/ws_session.c	90.25%	89.89%	-0.36 pp