fix(test): properly serialize env variable tests with serial_test crate

Add serial_test dependency and mark all tests that modify environment
variables with #[serial_test::serial] to prevent race conditions when
tests run in parallel.

Fixed tests:
- test_env_security_with_invalid_profile
- test_env_security_with_valid_profiles
- test_env_config_loading

This resolves the flaky test failure in CI by ensuring these tests
run one at a time instead of being ignored.

Author: avrabe

Cargo.lock

@@ -61,6 +61,7 @@ pulseengine-mcp-protocol = { workspace = true }
 tempfile = { workspace = true }
 assert_matches = { workspace = true }
 tokio-test = "0.4"
+serial_test = "3.0"
 
 [features]
 default = ["auto-config"]

mcp-security-middleware/Cargo.toml

@@ -473,6 +473,7 @@ mod tests {
     }
 
     #[test]
+    #[serial_test::serial]
     fn test_env_config_loading() {
         unsafe {
             env::set_var("MCP_SECURITY_PROFILE", "development");

mcp-security-middleware/src/config.rs

@@ -173,7 +173,7 @@ mod tests {
     }
 
     #[test]
-    #[ignore] // TODO: Fix race condition with parallel tests - use serial_test crate
+    #[serial_test::serial]
     fn test_env_security_with_invalid_profile() {
         use std::env;
 
@@ -192,6 +192,7 @@ mod tests {
     }
 
     #[test]
+    #[serial_test::serial]
     fn test_env_security_with_valid_profiles() {
         use std::env;