core: Fix Exception handling in XMLOutputParser (#19126)

- **Description:** 
  - Exception handling in `XMLOutputParser`
1. Add Exception handling at `root = ET.fromstring(text)` // raises
`ET.ParseError`
    2. Fix Exception class (commonly uses in `BaseOutputParser` class)
  - AS-IS: raise `ValueError`, `ET.ParserError` without handling
    ```python
    # langchain_core/output_parsers/xml.py

        text = text.strip()
        if (text.startswith("<") or text.startswith("\n<")) and (
            text.endswith(">") or text.endswith(">\n")
        ):
            root = ET.fromstring(text)
            return self._root_to_dict(root)
        else:
            raise ValueError(f"Could not parse output: {text}")
    ```
  - TO-BE: raise `OutputParserException`
    ```python
    # langchain_core/output_parsers/xml.py

        text = text.strip()
        if (text.startswith("<") or text.startswith("\n<")) and (
            text.endswith(">") or text.endswith(">\n")
        ):
            try:
                root = ET.fromstring(text)
                return self._root_to_dict(root)

            except ET.ParseError:
raise OutputParserException(f"Could not parse output: {text}")

        else:
raise OutputParserException(f"Could not parse output: {text}")

    ``` 
- **Issue:** #19107  
- **Dependencies:** None

libs/core/tests/unit_tests/output_parsers/test_xml_parser.py

@@ -1,6 +1,7 @@
 """Test XMLOutputParser"""
 import pytest
 
+from langchain_core.exceptions import OutputParserException
 from langchain_core.output_parsers.xml import XMLOutputParser
 
 DEF_RESULT_ENCODING = """<?xml version="1.0" encoding="UTF-8"?>
@@ -59,6 +60,6 @@ def test_xml_output_parser_fail(result: str) -> None:
 
     xml_parser = XMLOutputParser()
 
-    with pytest.raises(ValueError) as e:
+    with pytest.raises(OutputParserException) as e:
         xml_parser.parse(result)
-    assert "Could not parse output" in str(e)
+    assert "Failed to parse" in str(e)