## Motivation and Context
The ping utility says the receiver "MUST respond promptly with an empty response",
and a server may treat a missing answer as a stale connection and drop it. Both reference SDKs honor
that on the client side (the TypeScript protocol layer installs an automatic pong,
and the Python session answers `PingRequest` with an empty result), but this client answered
a server ping with Method not found over Streamable HTTP and ignored it entirely over stdio,
leaving long-lived legacy sessions vulnerable to a liveness-checking server.
Over HTTP, an unhandled `ping` is now answered with the empty result in `dispatch_server_request`.
The default deliberately does not live in `@server_request_handlers`: that registry's `any?` opens
the GET listening stream on connect, and a built-in pong must not change listener behavior -
a server can only ping on a stream that is already open. A handler registered via
`on_server_request("ping")` still wins.
Over stdio, the read loop now answers a ping frame inline and keeps waiting for its response.
Frames are told apart by shape rather than id: one carrying `method` is a request or notification,
never the awaited response, so a server-chosen ping id that collides with the awaited one is still answered -
and a request frame can no longer be returned to the caller as if it were the response.
Between requests nothing reads stdout, so a queued ping is answered when the next request starts reading,
which is inherent to the single-threaded stdio client. Other server-to-client requests over stdio stay
unsupported as documented.
The pong is best effort on both transports, in the ways the reference SDKs imply: a pong that cannot be written
over a broken stdin is dropped instead of failing the unrelated request whose response the loop is reading,
and a ping whose id is not a String or a Number - a shape the TypeScript and Python schemas reject outright -
is not answered with a pong: over stdio it is skipped entirely, and over HTTP it falls through to Method not found,
exactly as before this change.
The modern lifecycle removes `ping` altogether (SEP-2575), so this applies to handshake-lifecycle connections only.
## How Has This Been Tested?
New transport tests: over HTTP, a ping delivered on the SSE stream results in a POSTed empty-result response with
the ping's id, and a user-registered `"ping"` handler overrides the default; over stdio, a ping injected while
a response is awaited is answered on stdin and the original response still returns. Both fail without the change
(Method not found / no answer). Further tests cover the best-effort edges: a pong write failure over
a closed stdin leaves the awaited response intact, and a malformed-id ping is skipped over stdio
and answered with Method not found over HTTP. `bundle exec rake` (tests, RuboCop, and conformance baseline) passes.
## Breaking Changes
None. A server ping that previously received Method not found (or nothing) now receives the empty result
the specification requires.
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