* fix: handle unspecified column_configs for KNN

Fixes a bug in which we tried to find the embedding
columns. The prior integration tests for KNN all had
certain fields filled in and hence couldn't catch this
case. Thanks to the offline report from Amarnath Mullick.

* embedding_service is only available in SpannerVectorStore constructor

Co-authored-by: Averi Kitsch <[email protected]>

1) text-bison is no longer supported, switch to gemini-2.0-flash-001;
2) add allowed node types to restrict the number of different node
   types;

Co-authored-by: Averi Kitsch <[email protected]>

…om retrievers (#159)

* Add Spanner Graph QA Chain

* Formatted notebook. Added copyright message to prompts file.

* Add missing imports for random graph name

* Make input table name randomized in integration tests to avoid name collision for tests running parallely from different python environments

* Provide timeout to graph cleanup

* Make default timeout of 300 secs for ddl application

* Increase timeout of integration test

* Change integration test timeout

* Minor formatting fixes

* Make the ddl operations test fixture scoped for the module

* Addressed review comments

* Addressed a few other review comments.

* Remove unused function

* fix type check errors

* Addressed review comments

* Addressed review comments

* Clear default project id from notebook

* Add import statement for SpanerGraphQAChain to notebook

* Add retrievers for Spanner Graph RAG

* Add licence headers

* Fix DATABASE name key

* Fix lint error on import ordering

* Fix lint errors

* Few minor changes to the SpannerGraphNodeVectorRetriever

* Fix lint error

* Add an option to expand context graph by hops

* Fix lint error

* Addressed review comments

* Remove expansion query options

* Add backticks to property names

* Change copyright year

* Address review comments

* Rename the retrievers. Merge the semantic retriever with the gql retriever.

* Fixed lint errors

* Change vertex ai versionto latest

* Fix lint errors

* Add documentation. Fixes the case where expands_by_hops is 0

* Add unit test for expand_by_hops=0

* Fix formatting for documentation

* Addressed review comments

* minor change

* Minor change

* Unpin langchain-google-spanner library

* Addressed review comments

---------

Co-authored-by: Amarnath Mullick <[email protected]>
Co-authored-by: Averi Kitsch <[email protected]>

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>