fix(document_loaders/telegram): fix pandas calls + add tests (#4806)

# Fix Telegram API loader + add tests.
I was testing this integration and it was broken with next error:
```python
message_threads = loader._get_message_threads(df)
KeyError: False
```
Also, this particular loader didn't have any tests / related group in
poetry, so I added those as well.

@hwchase17 / @eyurtsev please take a look on this fix PR.

---------

Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

tests/unit_tests/document_loaders/parsers/test_public_api.py

@@ -0,0 +1,12 @@
+from langchain.document_loaders.parsers import __all__
+
+
+def test_parsers_public_api_correct() -> None:
+    """Test public API of parsers for breaking changes."""
+    assert set(__all__) == {
+        "PyPDFParser",
+        "PDFMinerParser",
+        "PyMuPDFParser",
+        "PyPDFium2Parser",
+        "PDFPlumberParser",
+    }