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>

pyproject.toml

@@ -86,6 +86,8 @@ lxml = {version = "^4.9.2", optional = true}
 pymupdf = {version = "^1.22.3", optional = true}
 pypdfium2 = {version = "^4.10.0", optional = true}
 gql = {version = "^3.4.1", optional = true}
+pandas = {version = "^2.0.1", optional = true}
+telethon = {version = "^1.28.5", optional = true}
 
 
 [tool.poetry.group.docs.dependencies]
@@ -190,6 +192,8 @@ extended_testing = [
  "pypdfium2",
  "tqdm",
  "lxml",
+ "pandas",
+ "telethon",
 ]
 
 [tool.ruff]