community: Cassandra Vector Store: modernize implementation (#27253)

**Description:** 

This PR updates `CassandraGraphVectorStore` to be based off
`CassandraVectorStore`, instead of using a custom CQL implementation.
This allows users using a `CassandraVectorStore` to upgrade to a
`GraphVectorStore` without having to change their database schema or
re-embed documents.

This PR also updates the documentation of the `GraphVectorStore` base
class and contains native async implementations for the standard graph
methods: `traversal_search` and `mmr_traversal_search` in
`CassandraVectorStore`.

**Issue:** No issue number.

**Dependencies:** #27078
(already-merged)

**Lint and test**: 
- Lint and tests all pass, including existing
`CassandraGraphVectorStore` tests.
- Also added numerous additional tests based of the tests in
`langchain-astradb` which cover many more scenarios than the existing
tests for `Cassandra` and `CassandraGraphVectorStore`

** BREAKING CHANGE**

Note that this is a breaking change for existing users of
`CassandraGraphVectorStore`. They will need to wipe their database table
and restart.

However:
- The interfaces have not changed. Just the underlying storage
mechanism.
- Any one using `langchain_community.vectorstores.Cassandra` can instead
use `langchain_community.graph_vectorstores.CassandraGraphVectorStore`
and they will gain Graph capabilities without having to re-embed their
existing documents. This is the primary goal of this PR.

---------

Co-authored-by: Erick Friis <[email protected]>

libs/community/langchain_community/graph_vectorstores/__init__.py

@@ -144,11 +144,13 @@
 from langchain_community.graph_vectorstores.links import (
     Link,
 )
+from langchain_community.graph_vectorstores.mmr_helper import MmrHelper
 
 __all__ = [
     "GraphVectorStore",
     "GraphVectorStoreRetriever",
     "Node",
     "Link",
     "CassandraGraphVectorStore",
+    "MmrHelper",
 ]