Fix and validate io_uring dispatcher before production use

[AlexJuca]

Context

The Linux io_uring dispatcher is intended as a high-performance backend, but it needs correctness validation before production use. The current implementation appears incomplete compared with the epoll/kqueue paths and should not be treated as production-ready without dedicated tests.

Relevant code:

• src/io/event_dispatcher_io_uring.c
• src/server.c (use-io-uring selection)
• server.conf

Scope

• Verify the io_uring server path submits initial accept/read operations correctly.
• Ensure client lifecycle is safe when completions arrive after close/free.
• Handle read, write, accept, timer, cancellation, and shutdown paths explicitly.
• Add tests or CI coverage that can run the server with use-io-uring true on Linux when liburing is available.
• Decide whether io_uring should be disabled by default until validated.

Acceptance criteria

• io_uring mode accepts connections and processes normal command traffic reliably.
• io_uring mode handles disconnects, malformed frames, and shutdown without use-after-free or leaks.
• CI or a documented Linux validation script exercises io_uring behavior.
• Production docs clearly state support level and requirements.