validate_tool_result() called jsonschema.validate() on every tools/call,
which rebuilds the validator class, re-checks the schema, and re-crawls
its \ each time — even though a tool's output schema doesn't change
between calls. Cache the compiled validator per tool name instead, and
invalidate the cache when list_tools() reports a changed schema for that
tool. Error message selection (best_match) is unchanged.

Fixes modelcontextprotocol#3133