feat(docs): add a public search route for the deprecated docs MCP server - #166

The earlier finding about unauthenticated access to paid embeddings remains open; this incremental change only addresses request-body buffering.

This pull request adds a public POST /docs/search endpoint for clients of the deprecated docs MCP server and separates document ranking from MCP response formatting. The route uses the existing search index, fixes the page limit, validates queries, and includes end-to-end coverage for unauthenticated search and invalid requests.