Add ShannonBase vector store integration

[Apoorva Verma (apoorva-01)]

Hey! This adds langchain-shannonbase, a vector store built on MySQL 9's native VECTOR type. The same code works on ShannonBase, plain self-hosted MySQL 9, and HeatWave, since they all share the VECTOR / STRING_TO_VECTOR / DISTANCE functions.

Why bother?
Right now the only MySQL vector store around is locked to Google Cloud SQL, so if your data's in plain MySQL 9 or ShannonBase there just wasn't one. ShannonBase even has "LangChain integration" sitting on their wishlist, so this scratches that itch.

A few notes:

• it's on PyPI: https://pypi.org/project/langchain-shannonbase/
• passes the standard VectorStoreIntegrationTests (runs offline in CI on 3.9 / 3.11 / 3.12)
• does metadata filtering, MMR, and get_by_ids

The PR just adds the doc page (src/oss/python/integrations/vectorstores/shannonbase.mdx) and a packages.yml entry. If it qualifies for highlight as a new integration that'd be lovely, but totally fine if not.

Thanks for taking a look!

[github-actions]

Thanks for opening a docs PR, Apoorva Verma (@apoorva-01)! When it's ready for review, please add the relevant reviewers:

• @mdrxy (Python integrations)

[langchain-oss-automated-triage]

Thanks for contributing a new integration docs page, Apoorva Verma (@apoorva-01)!

We receive a large number of PRs and review them as quickly as we can. Please bear with us as we work through the queue.

If you have already tagged a maintainer on this PR, do not tag them again.

[Apoorva Verma (apoorva-01)]

Mason Daugherty (@mdrxy) - It's ready for review whenever you get a chance. Thanks!