fix(test): add serial_test to all env variable tests across workspace

Systematically address potential race conditions in all tests that modify
environment variables by adding serial_test dependency and marking tests
with #[serial_test::serial].

Changes:
- mcp-security-middleware: 3 tests marked serial
- mcp-cli-derive: 1 test marked serial
- mcp-cli: 4 tests marked serial

This prevents flaky test failures in CI when tests run in parallel and
modify the same or overlapping environment variables.

Tests using unique env var names per test (like in mcp-auth) don't need
serialization as they don't conflict with each other.

Author: avrabe

Cargo.lock

@@ -33,3 +33,4 @@ clap = { version = "4.0", features = ["derive"] }
 serde = { version = "1.0", features = ["derive"] }
 tokio = { version = "1.0", features = ["macros", "rt-multi-thread"] }
 thiserror = { workspace = true }
+serial_test = "3.0"

mcp-cli-derive/Cargo.toml

@@ -358,6 +358,7 @@ mod integration_tests {
 
     /// Test environment variable support
     #[test]
+    #[serial_test::serial]
     fn test_env_var_support() {
         use std::env;
 

mcp-cli-derive/tests/test_mcp_config.rs

@@ -40,3 +40,4 @@ derive = ["pulseengine-mcp-cli-derive"]
 [dev-dependencies]
 tokio-test = "0.4"
 tempfile = "3.0"
+serial_test = "3.0"

mcp-cli/Cargo.toml

@@ -147,6 +147,7 @@ fn test_env_utils_get_env_or_default() {
 }
 
 #[test]
+#[serial_test::serial]
 fn test_env_utils_with_set_env_var() {
     use env_utils::*;
 
@@ -183,6 +184,7 @@ fn test_env_utils_get_required_env_missing() {
 }
 
 #[test]
+#[serial_test::serial]
 fn test_env_utils_get_required_env_present() {
     use env_utils::*;
 
@@ -204,6 +206,7 @@ fn test_env_utils_get_required_env_present() {
 }
 
 #[test]
+#[serial_test::serial]
 fn test_env_utils_get_required_env_invalid_type() {
     use env_utils::*;
 
@@ -231,6 +234,7 @@ fn test_env_utils_get_required_env_invalid_type() {
 }
 
 #[test]
+#[serial_test::serial]
 fn test_env_utils_get_required_env_valid_type() {
     use env_utils::*;