🧪 Add missing error path test for extract_manifest

🎯 **What:** Adds a missing test coverage for `extract_manifest` error path when required file paths do not exist.
📊 **Coverage:** Tests that missing upstream tools, commands or entrypoint file results in a clear `std::io::ErrorKind::NotFound` Error being surfaced without panics.
✨ **Result:** Improved robustness by asserting on standard Rust IO error variants when file sources cannot be read, enabling safer compatibility manifest loading logic across temporary directories and dynamic contexts.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: badMade

patch_pr.py

@@ -1,4 +1,3 @@
-import sys
 
 def replace_in_file(filepath, search_str, replace_str):
     with open(filepath, 'r') as f:

rust/Cargo.lock

@@ -12,3 +12,6 @@ runtime = { path = "../runtime" }
 
 [lints]
 workspace = true
+
+[dev-dependencies]
+tempfile = "3.27.0"

rust/crates/compat-harness/Cargo.toml

@@ -354,4 +354,13 @@ mod tests {
         assert!(names.contains(&"AgentTool"));
         assert!(names.contains(&"BashTool"));
     }
+
+    #[test]
+    fn extract_manifest_returns_not_found_for_invalid_paths() {
+        let temp = tempfile::tempdir().unwrap();
+        let paths = UpstreamPaths::from_repo_root(temp.path());
+        let result = extract_manifest(&paths);
+        let err = result.expect_err("should fail when files do not exist");
+        assert_eq!(err.kind(), std::io::ErrorKind::NotFound);
+    }
 }