Add opt-in BM25 tool search for large catalogs - #1449

Large tool catalogs consume model context and reduce tool-selection accuracy. This adds server-side discovery and a standalone search API for gateways, behind a dependency-free tool-search feature.

• Search index: Lazy BM25 Okapi ranking over names, titles, descriptions, and optional parameter metadata. Returns full tool definitions and scores; supports catalog replacement.
• Server wrapper: ToolSearchServer replaces discovery with search_tools / call_tool, supporting always-visible tools, configurable names, and a default five-result limit.
• Behavior: Original tools remain directly callable. Dispatch rejects synthetic-tool recursion and preserves metadata and retry state. Each listing/search fetches all catalog pages; the index rebuilds only when definitions change.
• Scope: Delegates non-tool handler behavior unchanged. Includes API documentation; optional regex search is deferred.

use rmcp::tool_search::{ToolSearchIndex, ToolSearchServer};

let index = ToolSearchIndex::new(tools);
let hits = index.search("create a github issue", 5);

// Alternatively, expose synthetic discovery tools:
let server = ToolSearchServer::new(my_handler);

Search controls discovery, not authorization; underlying handlers retain access enforcement.

• Fixes feat: built-in tool search (BM25) for large tool catalogs, like FastMCP's search_tools #1448