Issue #3586559: Add MariaDB VDB provider to matrix

Description

Adds MariaDB to the AI provider matrix/registry as a Vector Database provider.

This makes the existing MariaDB VDB provider discoverable alongside the other supported VDB providers such as Elasticsearch, Milvus, Pinecone, PostgreSQL, and SQLite.

Testing instructions

1. Reviewed the changed files and confirmed MariaDB is listed as a Vector Database provider.
2. Confirmed this is a documentation/provider-discovery metadata change only and does not introduce runtime behavior changes.

Checklist

• I have linked the related issue in the MR title or description
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

AI Compliance

Note

Check the one that best describes your usage, or leave all unchecked if AI was not significantly used.

• AI Assisted Code
  Mainly written by a human; AI used for autocomplete or partial generation under full human supervision.

• AI Generated Code
  Mainly generated by AI, reviewed and approved by a human before this MR was created.

• Vibe Coded
  Generated by AI and only functionally reviewed before this MR was created.

Closes #3586559