Fix QuestionListOutputParser (#9738)

This PR fixes `QuestionListOutputParser` text splitting.

`QuestionListOutputParser` incorrectly splits numbered list text into
lines. If text doesn't end with `\n` , the regex doesn't capture the
last item. So it always returns `n - 1` items, and
`WebResearchRetriever.llm_chain` generates less queries than requested
in the search prompt.

How to reproduce:

```python
from langchain.retrievers.web_research import QuestionListOutputParser

parser = QuestionListOutputParser()

good = parser.parse(
    """1. This is line one.
    2. This is line two.
    """  # <-- !
)

bad = parser.parse(
    """1. This is line one.
    2. This is line two."""    # <-- No new line.
)

assert good.lines == ['1. This is line one.\n', '2. This is line two.\n'], good.lines
assert bad.lines == ['1. This is line one.\n', '2. This is line two.'], bad.lines
```

NOTE: Last item will not contain a line break but this seems ok because
the items are stripped in the
`WebResearchRetriever.clean_search_query()`.

libs/langchain/langchain/retrievers/web_research.py

@@ -61,7 +61,7 @@ class QuestionListOutputParser(PydanticOutputParser):
         super().__init__(pydantic_object=LineList)
 
     def parse(self, text: str) -> LineList:
-        lines = re.findall(r"\d+\..*?\n", text)
+        lines = re.findall(r"\d+\..*?(?:\n|$)", text)
         return LineList(lines=lines)
 
 

libs/langchain/tests/unit_tests/retrievers/test_web_research.py

@@ -0,0 +1,36 @@
+from typing import List
+
+import pytest
+
+from langchain.retrievers.web_research import QuestionListOutputParser
+
+
+@pytest.mark.parametrize(
+    "text,expected",
+    (
+        (
+            "1. Line one.\n",
+            ["1. Line one.\n"],
+        ),
+        (
+            "1. Line one.",
+            ["1. Line one."],
+        ),
+        (
+            "1. Line one.\n2. Line two.\n",
+            ["1. Line one.\n", "2. Line two.\n"],
+        ),
+        (
+            "1. Line one.\n2. Line two.",
+            ["1. Line one.\n", "2. Line two."],
+        ),
+        (
+            "1. Line one.\n2. Line two.\n3. Line three.",
+            ["1. Line one.\n", "2. Line two.\n", "3. Line three."],
+        ),
+    ),
+)
+def test_list_output_parser(text: str, expected: List[str]) -> None:
+    parser = QuestionListOutputParser()
+    result = parser.parse(text)
+    assert result.lines == expected