Release PSA crypto state in uninitNetSystem(), not per socket (fixes #612)

[ClintonSarkar]

Summary

SocketMbedTLS::close() called mbedtls_psa_crypto_free() for every socket. That call is process-global: it tears down all PSA key slots and the shared entropy/RNG state, so closing one TLS socket breaks every other live TLS socket in the process. A process with ~6 concurrent TLS WebSocket clients reconnecting on their own threads hit PSA_ERROR_BAD_STATE failures and heap-corruption aborts. This PR moves the teardown into ix::uninitNetSystem(), as suggested in #612.

What changed

• close(): no longer calls mbedtls_psa_crypto_free().
• uninitNetSystem(): calls mbedtls_psa_crypto_free() on all platforms when built with mbedTLS >= 3.6, mirroring the existing WSAStartup/WSACleanup pair. Documented in IXNetSystem.h.
• initMBedTLS() guard: a per-socket _mbedtlsInitialized flag stops the ctor + init() double call from re-initialising live mbedTLS contexts (with MBEDTLS_THREADING_C that re-inits live mutexes). close() clears it so a socket can be re-initialised. psa_crypto_init() stays outside the guard (it is idempotent).
• Checked psa_crypto_init() in init(): a failed PSA init now fails the connect with PSA crypto init failed: <status> instead of running TLS on uninitialised PSA state.
• Test: new test/IXSocketMbedTLSPSATest.cpp ([socket_mbedtls], built only with USE_TLS + USE_MBED_TLS and mbedTLS >= 3.6). It probes PSA state via psa_generate_random and an HMAC key import (hashing works without PSA init, so it would not detect the bug), across another socket's close, and checks that uninitNetSystem() followed by psa_crypto_init() recovers.
• PSA include guard: IXNetSystem.cpp includes <psa/crypto.h> only for mbedTLS >= 3.6, so builds against mbedTLS 2.x (no psa/ headers) are unaffected.
• Placeholder below 3.6: the PSA test binary carries a single passing placeholder case below mbedTLS 3.6, so it never reports "no tests ran" and ctest stays green there.

Affected versions

Version	Affected (built with mbedTLS >= 3.6)
v11.4.5 and earlier	No
v11.4.6	Yes (introduced by #527)
v12.0.0	Yes (guard widened to mbedTLS 4.x by #579)
v12.0.1	Yes
master	Yes

Builds against mbedTLS < 3.6 never compiled the call and are not affected.

Behaviour change

Applications that never call ix::uninitNetSystem() now keep PSA state until process exit, which the OS reclaims anyway. Applications that do call it must do so after closing all sockets — the same contract as WSACleanup on Windows.

Note on threading

Concurrent TLS from multiple threads also requires mbedTLS built with MBEDTLS_THREADING_C, which the stock mbedTLS config leaves off. This is a documentation note only; this PR does not change it.

Testing

Linux (g++ 13, Ninja), mbedTLS 3.6.5 built from source, configured with -DUSE_TLS=ON -DUSE_MBED_TLS=ON -DUSE_ZLIB=OFF -DUSE_TEST=ON.

Red — test commit only, on unfixed master (IXSocketMbedTLSPSATest "[socket_mbedtls]"):

psa_generate_random failed: -137
...
SocketMbedTLS close does not tear down process-global PSA state
...
test/IXSocketMbedTLSPSATest.cpp:64: FAILED:
  REQUIRE( psaStateWorks() )
with expansion:
  false
...
test cases: 2 | 1 passed | 1 failed
assertions: 6 | 5 passed | 1 failed

(-137 is PSA_ERROR_BAD_STATE; line 64 is the check right after another socket's close().)

Green — with the fix:

All tests passed (7 assertions in 2 test cases)

Full ctest: 17/18 passed. IXHttpServerTest fails on Accept-Encoding == "gzip" both with and without this change (my build had USE_ZLIB=OFF), so it is unrelated.

The pre-3.6 branches (include guard and test placeholder) were syntax-checked by forcing the version condition off with g++ -fsyntax-only, not built against a real older mbedTLS.

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