perf: use set for O(1) membership check in TestFiles.add()

The previous implementation used `if test_file not in self.test_files`
which performs O(n) equality comparison across all TestFile objects.
Replace with a `_seen_paths` set keyed on `instrumented_behavior_file_path`
for O(1) deduplication lookups.

Author: KRRT7

codeflash/models/models.py

@@ -426,9 +426,15 @@ class TestFile(BaseModel):
 
 class TestFiles(BaseModel):
     test_files: list[TestFile]
+    _seen_paths: set[Path] = PrivateAttr(default_factory=set)
+
+    def model_post_init(self, __context: Any, /) -> None:
+        self._seen_paths = {tf.instrumented_behavior_file_path for tf in self.test_files}
 
     def add(self, test_file: TestFile) -> None:
-        if test_file not in self.test_files:
+        key = test_file.instrumented_behavior_file_path
+        if key not in self._seen_paths:
+            self._seen_paths.add(key)
             self.test_files.append(test_file)
         else:
             msg = "Test file already exists in the list"