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/client.rs

@@ -1040,3 +1040,5 @@ impl RunningClientHandler {
 #[cfg(test)]
 #[path = "client_security_tests.rs"]
 mod security_tests;
+#[path = "client_adversarial_tests.rs"]
+mod adversarial_tests;