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/test_csv_loader.py

@@ -0,0 +1,89 @@
+from pathlib import Path
+
+from langchain.docstore.document import Document
+from langchain.document_loaders.csv_loader import CSVLoader
+
+
+class TestCSVLoader:
+    # Tests that a CSV file with valid data is loaded successfully.
+    def test_csv_loader_load_valid_data(self) -> None:
+        # Setup
+        file_path = self._get_csv_file_path("test_nominal.csv")
+        expected_docs = [
+            Document(
+                page_content="column1: value1\ncolumn2: value2\ncolumn3: value3",
+                metadata={"source": file_path, "row": 0},
+            ),
+            Document(
+                page_content="column1: value4\ncolumn2: value5\ncolumn3: value6",
+                metadata={"source": file_path, "row": 1},
+            ),
+        ]
+
+        # Exercise
+        loader = CSVLoader(file_path=file_path)
+        result = loader.load()
+
+        # Assert
+        assert result == expected_docs
+
+    # Tests that an empty CSV file is handled correctly.
+    def test_csv_loader_load_empty_file(self) -> None:
+        # Setup
+        file_path = self._get_csv_file_path("test_empty.csv")
+        expected_docs: list = []
+
+        # Exercise
+        loader = CSVLoader(file_path=file_path)
+        result = loader.load()
+
+        # Assert
+        assert result == expected_docs
+
+    # Tests that a CSV file with only one row is handled correctly.
+    def test_csv_loader_load_single_row_file(self) -> None:
+        # Setup
+        file_path = self._get_csv_file_path("test_one_row.csv")
+        expected_docs = [
+            Document(
+                page_content="column1: value1\ncolumn2: value2\ncolumn3: value3",
+                metadata={"source": file_path, "row": 0},
+            )
+        ]
+
+        # Exercise
+        loader = CSVLoader(file_path=file_path)
+        result = loader.load()
+
+        # Assert
+        assert result == expected_docs
+
+    # Tests that a CSV file with only one column is handled correctly.
+    def test_csv_loader_load_single_column_file(self) -> None:
+        # Setup
+        file_path = self._get_csv_file_path("test_one_col.csv")
+        expected_docs = [
+            Document(
+                page_content="column1: value1",
+                metadata={"source": file_path, "row": 0},
+            ),
+            Document(
+                page_content="column1: value2",
+                metadata={"source": file_path, "row": 1},
+            ),
+            Document(
+                page_content="column1: value3",
+                metadata={"source": file_path, "row": 2},
+            ),
+        ]
+
+        # Exercise
+        loader = CSVLoader(file_path=file_path)
+        result = loader.load()
+
+        # Assert
+        assert result == expected_docs
+
+    # utility functions
+    def _get_csv_file_path(self, file_name: str) -> str:
+        return str(Path(__file__).resolve().parent / "test_docs" / "csv" / file_name)