Client connection reuse and server-side query cancellation

[joe-clickhouse]

Summary

The goal of this PR is to harden the MCP server for heavy, sustained usage by reusing client connections, adding real server-side query cancellation, and preventing worker thread pool exhaustion from zombie queries.

• Connection reuse: Cache clickhouse_connect clients by config key instead of creating a new client on every tool call. Eliminates hundreds of ms of overhead per call that a significant contributor to intermittent timeouts under sustained use.
• Server-side query cancellation: Assign a query_id to every query and issue KILL QUERY on timeout instead of the no-op future.cancel(). Timed-out queries no longer continue running as zombies consuming worker threads and ClickHouse server resources.
• Timeout alignment: Auto-cap send_receive_timeout to query_timeout + 5, unless explicitly overridden so worker threads unblock shortly after the MCP timeout fires, preventing thread pool exhaustion.
• Stale client eviction: Evict cached clients on connection errors and on failed liveness pings after idle. Read-only metadata calls like list_databases and list_tables retry once with a fresh client. run_query evicts but does not retry because writes could duplicate.
• Config resolution fix: Resolve session config overrides on the request thread where FastMCP ContextVar is available before dispatching to the worker thread, fixing a latent bug where PR Client config override support via MCP Context Session states #115 overrides were silently missed inside the executor.