fix(test): prevent flaky test_caching test with serial execution

The test_caching test uses a static cache shared across all tests. When
tests run in parallel, other tests can modify the cache between
clear_cache() and the cache_size() assertions, causing spurious failures.

Fix by adding #[serial] attribute from serial_test crate to ensure this
test runs in isolation from other tests that use the cache.

Author: echobt

Cargo.lock

@@ -132,3 +132,4 @@ tempfile = { workspace = true }
 tokio-test = { workspace = true }
 wiremock = { workspace = true }
 insta = { workspace = true }
+serial_test = { workspace = true }

src/cortex-engine/Cargo.toml

@@ -247,6 +247,7 @@ pub fn git_root(path: &Path) -> Option<PathBuf> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use serial_test::serial;
     use tempfile::TempDir;
 
     fn setup_test_dir() -> TempDir {
@@ -344,8 +345,9 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn test_caching() {
-        // Clear cache first since tests run in parallel and share static cache
+        // Clear cache first - must use #[serial] since tests share static cache
         clear_cache();
         assert_eq!(cache_size(), 0);