fix: support date comparison operators (>=, <=, >, <) in metadata filtering (#12982)

## Description

This PR fixes the issue where date metadata conditions with comparison
operators (`>=`, `<=`, `>`, `<`) did not work correctly in the
`/api/v1/retrieval` endpoint.

  ## Problem

  When using metadata conditions like:
  ```json
  {
    "metadata_condition": {
      "conditions": [
        {
          "name": "date",
          "comparison_operator": ">=",
          "value": "2027-01-13"
        }
      ]
    }
  }

  The filtering did not work as expected because:
  1. Operators >= and <= were not mapped to internal symbols ≥ and ≤
2. Date strings like "2027-01-13" failed to parse with
ast.literal_eval()
  3. Non-standard date formats were incorrectly compared as strings

  Solution

  Changes in common/metadata_utils.py:

  1. Added operator mapping in convert_conditions():
    - >= → ≥
    - <= → ≤
    - != → ≠
  2. Implemented strict date format detection in meta_filter():
- Only processes dates in YYYY-MM-DD format (10 characters, properly
formatted)
- When query value is a date, only matches data in the same standard
format
    - Non-standard formats (e.g., "2026年1月13日", "2026-1-22") are skipped
  3. Maintained backward compatibility:
    - Numeric comparisons still work
    - String comparisons still work
    - Only affects date-formatted queries

  Testing

  All test cases pass (8/8):
  - ✅ Date >= comparison
  - ✅ Date > comparison
  - ✅ Date < comparison
  - ✅ Date <= comparison
  - ✅ Date = comparison
  - ✅ Date range queries
  - ✅ Non-date string comparison (backward compatibility)
  - ✅ Numeric comparison (backward compatibility)

  Example Usage

  {
    "dataset_ids": ["xxx"],
    "question": "test",
    "metadata_condition": {
      "conditions": [
        {
          "name": "date",
          "comparison_operator": ">=",
          "value": "2027-01-13"
        }
      ]
    }
  }

  Notes

  - Only supports standard YYYY-MM-DD format
- Non-standard date formats in data are treated as data quality issues
and will not match
  - Users should ensure their date metadata is in the correct format

---------

Co-authored-by: Clint-chan <Clint-chan@users.noreply.github.com>

common/metadata_utils.py

@@ -24,7 +24,10 @@ def convert_conditions(metadata_condition):
         metadata_condition = {}
     op_mapping = {
         "is": "=",
-        "not is": "≠"
+        "not is": "≠",
+        ">=": "≥",
+        "<=": "≤",
+        "!=": "≠"
     }
     return [
         {
@@ -44,17 +47,59 @@ def meta_filter(metas: dict, filters: list[dict], logic: str = "and"):
         for input, docids in v2docs.items():
 
             if operator in ["=", "≠", ">", "<", "≥", "≤"]:
-                try:
-                    if isinstance(input, list):
-                        input = input[0]
-                    input = ast.literal_eval(input)
-                    value = ast.literal_eval(value)
-                except Exception:
-                    pass
-            if isinstance(input, str):
-                input = input.lower()
-            if isinstance(value, str):
-                value = value.lower()
+                # Check if input is in YYYY-MM-DD date format
+                input_str = str(input).strip()
+                value_str = str(value).strip()
+
+                # Strict date format detection: YYYY-MM-DD (must be 10 chars with correct format)
+                is_input_date = (
+                    len(input_str) == 10 and
+                    input_str[4] == '-' and
+                    input_str[7] == '-' and
+                    input_str[:4].isdigit() and
+                    input_str[5:7].isdigit() and
+                    input_str[8:10].isdigit()
+                )
+
+                is_value_date = (
+                    len(value_str) == 10 and
+                    value_str[4] == '-' and
+                    value_str[7] == '-' and
+                    value_str[:4].isdigit() and
+                    value_str[5:7].isdigit() and
+                    value_str[8:10].isdigit()
+                )
+
+                if is_value_date:
+                    # Query value is in date format
+                    if is_input_date:
+                        # Data is also in date format: perform date comparison
+                        input = input_str
+                        value = value_str
+                    else:
+                        # Data is not in date format: skip this record (no match)
+                        continue
+                else:
+                    # Query value is not in date format: use original logic
+                    try:
+                        if isinstance(input, list):
+                            input = input[0]
+                        input = ast.literal_eval(input)
+                        value = ast.literal_eval(value)
+                    except Exception:
+                        pass
+
+                    # Convert strings to lowercase
+                    if isinstance(input, str):
+                        input = input.lower()
+                    if isinstance(value, str):
+                        value = value.lower()
+            else:
+                # Non-comparison operators: maintain original logic
+                if isinstance(input, str):
+                    input = input.lower()
+                if isinstance(value, str):
+                    value = value.lower()
 
             matched = False
             try: