fix(cortex-common): fix flaky signal_safety and cwd_guard tests (#478)

The tests were failing intermittently due to shared mutable state
(static atomic variables and current working directory) being modified
by multiple tests running in parallel.

Fixes:
- Added serial_test dependency to cortex-common dev-dependencies
- Added #[serial] attribute to all signal_safety tests to prevent
  parallel execution of tests that use global signal handling state
- Added #[serial] attribute to all cwd_guard tests to prevent
  parallel execution of tests that modify the current working directory
- Fixed test_cleanup_guard by separating guard unwrap from assertion
  to keep the guard alive while testing lock exclusivity

Co-authored-by: Droid Agent <droid@factory.ai>

Author: echobt

Cargo.lock

@@ -38,3 +38,4 @@ async = ["tokio"]
 
 [dev-dependencies]
 tempfile = { workspace = true }
+serial_test = { workspace = true }

cortex-common/Cargo.toml

@@ -306,9 +306,11 @@ pub fn validate_directory(dir: impl AsRef<Path>) -> io::Result<()> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use serial_test::serial;
     use tempfile::TempDir;
 
     #[test]
+    #[serial]
     fn test_cwd_guard_restores() {
         let original = env::current_dir().unwrap();
         let temp_dir = TempDir::new().unwrap();
@@ -326,6 +328,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_cwd_guard_restores_on_panic() {
         let original = env::current_dir().unwrap();
         let temp_dir = TempDir::new().unwrap();
@@ -341,6 +344,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_cwd_guard_disabled() {
         let original = env::current_dir().unwrap();
         let temp_dir = TempDir::new().unwrap();
@@ -359,6 +363,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_cwd_guard_restore_now() {
         let original = env::current_dir().unwrap();
         let temp_dir = TempDir::new().unwrap();
@@ -373,6 +378,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_in_directory() {
         let original = env::current_dir().unwrap();
         let temp_dir = TempDir::new().unwrap();
@@ -385,6 +391,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_in_directory_result() {
         let original = env::current_dir().unwrap();
         let temp_dir = TempDir::new().unwrap();
@@ -399,6 +406,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_save_current() {
         let original = env::current_dir().unwrap();
         let temp_dir = TempDir::new().unwrap();

cortex-common/src/cwd_guard.rs

@@ -171,13 +171,15 @@ impl Drop for SignalCleanupGuard {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use serial_test::serial;
 
     fn reset_state() {
         release_signal_lock();
         reset_signal_count();
     }
 
     #[test]
+    #[serial]
     fn test_try_acquire_signal_lock() {
         reset_state();
 
@@ -195,6 +197,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_signal_count() {
         reset_state();
 
@@ -211,6 +214,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_should_force_exit() {
         reset_state();
 
@@ -229,18 +233,23 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_cleanup_guard() {
         reset_state();
 
         // First guard should succeed
         {
             let guard = SignalCleanupGuard::try_new();
             assert!(guard.is_some());
-            assert!(guard.unwrap().owns_lock());
+            let guard = guard.unwrap();
+            assert!(guard.owns_lock());
 
             // While guard is alive, another should fail
             let guard2 = SignalCleanupGuard::try_new();
             assert!(guard2.is_none());
+
+            // Keep guard alive until end of scope
+            drop(guard);
         }
 
         // After guard is dropped, new guard should succeed
@@ -251,6 +260,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_safe_signal_handler() {
         reset_state();