core: fix CommaSeparatedListOutputParser to handle columns that may contain commas in it (#26365)

- **Description:**
Currently CommaSeparatedListOutputParser can't handle strings that may
contain commas within a column. It would parse any commas as the
delimiter.
Ex. 
"foo, foo2", "bar", "baz"

It will create 4 columns: "foo", "foo2", "bar", "baz"

This should be 3 columns:

"foo, foo2", "bar", "baz"

- **Dependencies:**
Added 2 additional imports, but they are built in python packages.

import csv
from io import StringIO

- **Twitter handle:** @jkyamog

- [ ] **Add tests and docs**: 
1. added simple unit test test_multiple_items_with_comma

---------

Co-authored-by: Erick Friis <erick@langchain.dev>
Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>
Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/core/langchain_core/output_parsers/list.py

@@ -1,9 +1,11 @@
 from __future__ import annotations
 
+import csv
 import re
 from abc import abstractmethod
 from collections import deque
 from collections.abc import AsyncIterator, Iterator
+from io import StringIO
 from typing import Optional as Optional
 from typing import TypeVar, Union
 
@@ -162,7 +164,14 @@ class CommaSeparatedListOutputParser(ListOutputParser):
         Returns:
             A list of strings.
         """
-        return [part.strip() for part in text.split(",")]
+        try:
+            reader = csv.reader(
+                StringIO(text), quotechar='"', delimiter=",", skipinitialspace=True
+            )
+            return [item for sublist in reader for item in sublist]
+        except csv.Error:
+            # keep old logic for backup
+            return [part.strip() for part in text.split(",")]
 
     @property
     def _type(self) -> str:

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

@@ -64,6 +64,25 @@ def test_multiple_items() -> None:
     assert list(parser.transform(iter([text]))) == [[a] for a in expected]
 
 
+def test_multiple_items_with_comma() -> None:
+    """Test that a string with multiple comma-separated items with 1 item containing a
+    comma is parsed to a list."""
+    parser = CommaSeparatedListOutputParser()
+    text = '"foo, foo2",bar,baz'
+    expected = ["foo, foo2", "bar", "baz"]
+
+    assert parser.parse(text) == expected
+    assert add(parser.transform(t for t in text)) == expected
+    assert list(parser.transform(t for t in text)) == [[a] for a in expected]
+    assert list(parser.transform(t for t in text.splitlines(keepends=True))) == [
+        [a] for a in expected
+    ]
+    assert list(
+        parser.transform(" " + t if i > 0 else t for i, t in enumerate(text.split(" ")))
+    ) == [[a] for a in expected]
+    assert list(parser.transform(iter([text]))) == [[a] for a in expected]
+
+
 def test_numbered_list() -> None:
     parser = NumberedListOutputParser()
     text1 = (