feature: added warning when duckdb is used as a vectorstore without pandas (#30435)

added warning when duckdb is used as a vectorstore without pandas being
installed (currently used for similarity search result processing)

Thank you for contributing to LangChain!

- [ ] **PR title**: "community: added warning when duckdb is used as a
vectorstore without pandas"


- [ ] **PR message**: ***Delete this entire checklist*** and replace
with
- **Description:** displays a warning when using duckdb as a vector
store without pandas being installed, as it is used by the
`similarity_search` function
    - **Issue:** #29933 
    - **Dependencies:** None

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/community/langchain_community/vectorstores/duckdb.py

@@ -99,6 +99,7 @@ class DuckDB(VectorStore):
                 "Could not import duckdb package. "
                 "Please install it with `pip install duckdb`."
             )
+
         self.duckdb = duckdb
         self._embedding = embedding
         self._vector_key = vector_key
@@ -209,6 +210,11 @@ class DuckDB(VectorStore):
         Returns:
             A list of Documents most similar to the query.
         """
+        try:
+            import pandas as pandas
+        except ImportError:
+            warnings.warn("You may need to `pip install pandas` to use this method.")
+
         embedding = self._embedding.embed_query(query)  # type: ignore
         list_cosine_similarity = self.duckdb.FunctionExpression(
             "list_cosine_similarity",