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>

test/unit_test/common/test_metadata_filter_operators.py

@@ -33,6 +33,20 @@ def test_not_in_operator():
     assert meta_filter(metas, filters) == ["doc3"]
 
 
+def test_in_operator_with_list_value_is_case_insensitive():
+    metas = {"product": {"F2": ["doc1"], "F11": ["doc2"], "G1": ["doc3"]}}
+    filters = [{"key": "product", "op": "in", "value": ["F2", "F11"]}]
+
+    assert set(meta_filter(metas, filters)) == {"doc1", "doc2"}
+
+
+def test_not_in_operator_with_list_value_is_case_insensitive():
+    metas = {"product": {"F2": ["doc1"], "F11": ["doc2"], "G1": ["doc3"]}}
+    filters = [{"key": "product", "op": "not in", "value": ["F2", "F11"]}]
+
+    assert meta_filter(metas, filters) == ["doc3"]
+
+
 def test_start_with():
     # returns chunk where the metadata starts with the value
     metas = {"name": {"prefix_value": ["doc1"], "other": ["doc2"]}}