feat(rag): expose MCP document search tool

[JerTheBear2018]

Summary

Adds MCP RAG document search support so the agent can search indexed documents directly through the manage_rag tool. This extends RAG from only managing indexes to also retrieving grounded document context.

Linked Issue

Related to #1707

Type of Change

• New feature
• Bug fix
• Refactor
• Documentation

Checklist

• I searched existing issues and PRs for duplicates.
• I added tests for this change.
• Existing tests continue to pass.

How to Test

1. Run:

   pytest -q tests/test_rag_server_search.py tests/test_rag_mcp_search_schema.py tests/test_rag_search_signature.py tests/test_rag_keyword_fallback_owner.py

[JerTheBear2018]

All checks are passing. PR is ready for maintainer merge. Auto-merge is blocked because my account lacks EnablePullRequestAutoMerge permission.