[LangChain] Fix testing Jupyter Notebook vector_search.ipynb

[amotl]

About

Building upon the test infrastructure added with GH-97, this patch intends to fix a few last flaws within the Jupyter Notebook vector_search.ipynb, primarily to make the software tests succeed.

It also invokes corresponding tests on CI/GHA now.

/cc @ckurze, @WalBeh

[amotl]

Problem

## inserted before /cells/27/outputs/0:
+  output:
+    output_type: stream
+    name: stderr
+    text:
+      Collection not found

-- https://github.com/crate/cratedb-examples/actions/runs/6686252645/job/18165405211?pr=98#step:5:558

References

• Vector Store: "Collection not found" when using pre_delete_collection=True crate-workbench/langchain#5 (comment)

[amotl]

Works. 🚀

test.py::test_notebook[document_loader.ipynb]
test.py::test_notebook[vector_search.ipynb]
test.py::test_notebook[conversational_memory.ipynb]

-- https://github.com/crate/cratedb-examples/actions/runs/6686545401/job/18166024142?pr=98#step:5:470