* [Server] Let resources/read ask for input, and stop caching retries
The MRTR pattern names three methods and resources/read was the one whose
handler dropped an InputRequiredResult into the resource formatter. A
retry's answer also carried ttlMs and cacheScope, which the caching rules
forbid: its inputs are not part of any cache key.
* [Server] Refuse an ask the client cannot answer
A handler could put an elicitation into inputRequests for a client that
never declared one, and the only symptom was a retry that never came.
Checked against the request's own capabilities now, and reported as -32021
with the missing set — which is the code the spec defines for exactly this.
* [Server] Hand multi round-trip answers back typed
response() returned whatever JSON arrived, so every handler re-implemented
the same array poking. A malformed answer now reads as absent, which sends
the handler back to asking rather than to failing.
* [Server] Gate streamed input_required asks by capability too
checkInputRequests() only ran on the non-streaming path, so a handler
that emits a notification before returning InputRequiredResult could
still leak an undeclared-capability ask under HTTP 200.
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