add Solr vector_io provider

[mwcz]

Description

This adds a llama-stack vector_io provider for performing RAG lookups from Solr search indexes, supporting keyword, vector, and hybrid search. Developed by @Anxhela21 and myself.

See the solr provider README for a full description.

The solr provider was initially developed as a standalone provider, before the requirement to integrate with lightspeed-stack was known. Some signs of that remain, like having its own pyproject.toml and independent test suite. I'm happy to remove any of that which needs to go before merging.

Type of change

• Refactor
• New feature
• Bug fix
• CVE fix
• Optimization
• Documentation Update
• Configuration Update
• Bump-up service version
• Bump-up dependent library
• Bump-up library or tool used for development (does not change the final image)
• CI configuration change
• Unit tests improvement

Tools used to create PR

Identify any AI code assistants used in this PR (for transparency and review context)

• Assisted-by: Claude
• Generated by: Claude Code produced some of the initial code, followed by a lot of manual refactoring.

Related Tickets & Documents

• Related Issue #
• Closes #

Checklist before requesting a review

• I have performed a self-review of my code.
• PR has passed all pre-merge test jobs.
• If it is a core feature, I have added thorough tests.

Testing

• Please provide detailed steps to perform tests related to this code change.

TBD

• How were the fix/results from this change verified? Please provide relevant screenshots or results.

Full end to end tests were performed in collaboration with the RHOKP team, Lightspeed Core team, and OpenShift Lightspeed team. The recording may be available somewhere internally.

The Solr provider also has its own test suite.

Here is a screenshot of a lightspeed-stack query that performs RAG lookup using the Solr provider and RHOKP Solr index.