Add adversarial tests for client, handshake, masking, and relay modules

- Introduced `client_adversarial_tests.rs` to stress test connection limits and IP tracker race conditions.
- Added `handshake_adversarial_tests.rs` for mutational bit-flipping tests and timing neutrality checks.
- Created `masking_adversarial_tests.rs` to validate probing indistinguishability and SSRF prevention.
- Implemented `relay_adversarial_tests.rs` to ensure HOL blocking prevention and data quota enforcement.
- Updated respective modules to include new test paths.

src/proxy/handshake.rs

@@ -971,6 +971,10 @@ mod security_tests;
 #[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;
+
 /// 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.