Add security tests for connection limits and handshake integrity

- Implement a test to ensure that exceeding the user connection limit does not leak the current connections counter.
- Add tests for direct relay connection refusal and adversarial scenarios to verify proper error handling.
- Introduce fuzz testing for MTProto handshake to ensure robustness against malformed inputs and replay attacks.
- Remove obsolete short TLS probe throttle tests and integrate their functionality into existing security tests.
- Enhance middle relay tests to validate behavior during connection drops and cutovers, ensuring graceful error handling.
- Add a test for half-close scenarios in relay to confirm bidirectional data flow continues as expected.

src/proxy/handshake.rs

@@ -967,14 +967,14 @@ pub fn encrypt_tg_nonce(nonce: &[u8; HANDSHAKE_LEN]) -> Vec<u8> {
 #[path = "handshake_security_tests.rs"]
 mod security_tests;
 
-#[cfg(test)]
-#[path = "handshake_gap_short_tls_probe_throttle_security_tests.rs"]
-mod gap_short_tls_probe_throttle_security_tests;
-
 #[cfg(test)]
 #[path = "handshake_adversarial_tests.rs"]
 mod adversarial_tests;
 
+#[cfg(test)]
+#[path = "handshake_fuzz_security_tests.rs"]
+mod fuzz_security_tests;
+
 /// Compile-time guard: HandshakeSuccess holds cryptographic key material and
 /// must never be Copy.  A Copy impl would allow silent key duplication,
 /// undermining the zeroize-on-drop guarantee.