fix(#14389): normalize list metadata values for in filters (#14410)

## Summary
- normalize string items for list-valued metadata filters in
`meta_filter`
- fix `in` / `not in` case asymmetry when document metadata is
lowercased but filter list values are not
- add regression tests that cover the original issue scenario using
uppercase list values

## Validation
- `PYTHONPATH=external/ragflow pytest
external/ragflow/test/unit_test/common/test_metadata_filter_operators.py
-q`

## Notes
- I commented on #14389 before opening this PR to claim the issue.
- The new tests use `value=["F2", "F11"]` so they fail on the old
implementation and pass with this fix.
- This also benefits other non-comparison operators that flow through
the same normalization path.

Co-authored-by: copizza <copizza@users.noreply.github.com>
Co-authored-by: Wang Qi <wangq8@outlook.com>

common/metadata_utils.py

@@ -42,6 +42,13 @@ def convert_conditions(metadata_condition):
 def meta_filter(metas: dict, filters: list[dict], logic: str = "and"):
     doc_ids = set([])
 
+    def normalize_string_values(value):
+        if isinstance(value, str):
+            return value.lower()
+        if isinstance(value, list):
+            return [item.lower() if isinstance(item, str) else item for item in value]
+        return value
+
     def filter_out(v2docs, operator, value):
         ids = []
         for input, docids in v2docs.items():
@@ -96,14 +103,8 @@ def meta_filter(metas: dict, filters: list[dict], logic: str = "and"):
                         value = value.lower()
             else:
                 # Non-comparison operators: maintain original logic
-                if isinstance(input, str):
-                    input = input.lower()
-                elif operator in ("in", "not in") and isinstance(input, list):
-                    input = [x.lower() if isinstance(x, str) else x for x in input]
-                if isinstance(value, str):
-                    value = value.lower()
-                elif operator in ("in", "not in") and isinstance(value, list):
-                    value = [x.lower() if isinstance(x, str) else x for x in value]
+                input = normalize_string_values(input)
+                value = normalize_string_values(value)
 
             matched = False
             try: