Strips leading/trailing whitespace before parsing xml (#12297)

**Description:** When llms output leading or trailing whitespace for xml
(when using XMLOutputParser) the parser would raise a `ValueError: Could
not parse output: ...`. However, leading or trailing whitespace are
"ignorable" in the sense of XML standard.

**Issue:** I did not find an issue related.

**Dependencies:** None

**Tag maintainer:**

**Twitter handle:** donatoaz

Please make sure your PR is passing linting and testing before
submitting. Run `make format`, `make lint` and `make test` to check this
locally.

Done, updated unit test and ran `make docker_test`.

libs/langchain/langchain/output_parsers/xml.py

@@ -22,6 +22,8 @@ class XMLOutputParser(BaseOutputParser):
         encoding_match = self.encoding_matcher.search(text)
         if encoding_match:
             text = encoding_match.group(2)
+
+        text = text.strip()
         if (text.startswith("<") or text.startswith("\n<")) and (
             text.endswith(">") or text.endswith(">\n")
         ):

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

@@ -4,7 +4,7 @@ import pytest
 from langchain.output_parsers.xml import XMLOutputParser
 
 DEF_RESULT_ENCODING = """<?xml version="1.0" encoding="UTF-8"?>
-<foo>
+ <foo>
     <bar>
         <baz></baz>
         <baz>slim.shady</baz>