Fix model file list cache never being reused

cache_model_file_list_ compared os.path.getmtime(folder) (a float)
against model_file_list_cache[1] (the dirs dict), which is always
unequal, so the check returned None on every call and the model tree
was re-walked on each /experiment/models request.

The top-level folder was also never recorded in dirs, so drop the
broken scalar comparison and instead seed the folder's own mtime into
dirs (matching folder_paths.recursive_search) so it is validated by the
existing per-directory loop and direct additions still invalidate the cache.

app/model_manager.py

@@ -125,8 +125,6 @@ class ModelFileManager:
             return None
         if not os.path.isdir(folder):
             return None
-        if os.path.getmtime(folder) != model_file_list_cache[1]:
-            return None
         for x in model_file_list_cache[1]:
             time_modified = model_file_list_cache[1][x]
             folder = x
@@ -146,6 +144,11 @@ class ModelFileManager:
         result: list[str] = []
         dirs: dict[str, float] = {}
 
+        try:
+            dirs[directory] = os.path.getmtime(directory)
+        except FileNotFoundError:
+            logging.warning(f"Warning: Unable to access {directory}. Skipping this path.")
+
         for dirpath, subdirs, filenames in os.walk(directory, followlinks=True, topdown=True):
             subdirs[:] = [d for d in subdirs if d not in excluded_dir_names]
             if not include_hidden_files:

tests-unit/app_test/model_file_cache_test.py

@@ -0,0 +1,29 @@
+import os
+
+from app.model_manager import ModelFileManager
+
+
+class TestModelFileListCache:
+    def test_cache_is_reused_when_unchanged(self, tmp_path):
+        sub = tmp_path / "sub"
+        sub.mkdir()
+        (sub / "m.safetensors").write_bytes(b"x")
+
+        mgr = ModelFileManager()
+        out = mgr.recursive_search_models_(str(tmp_path), 0)
+        mgr.set_cache(str(tmp_path), out)
+
+        # nothing changed, so the cached result must be returned instead of None
+        assert mgr.cache_model_file_list_(str(tmp_path)) is out
+
+    def test_cache_invalidated_when_top_folder_changes(self, tmp_path):
+        mgr = ModelFileManager()
+        out = mgr.recursive_search_models_(str(tmp_path), 0)
+        mgr.set_cache(str(tmp_path), out)
+        assert mgr.cache_model_file_list_(str(tmp_path)) is out
+
+        # a file added directly under the top folder changes its mtime, which must
+        # invalidate the cache (the top folder is tracked, not just subfolders)
+        bumped = os.path.getmtime(str(tmp_path)) + 100
+        os.utime(str(tmp_path), (bumped, bumped))
+        assert mgr.cache_model_file_list_(str(tmp_path)) is None