tools/list and tools/call on main already admit `_meta` — MCP reserves it
for request metadata and a client may attach it to any request. ping still
refused every parameter, so a client that sends `_meta` on everything
(Codex does) got -32602 on its keepalive.
Rebased onto main and narrowed: when this branch was first written the same
gap existed on tools/list, which has since been fixed upstream. Only the
ping handler is left, so only the ping handler is touched.
Refs #2810
