[Server] Serve ClientGateway::elicit() under the modern lifecycle

[chr-hertel]

Building a server against the SDK meant picking an era: ClientGateway::elicit() on the handshake leg, or returning an InputRequiredResult and reading RequestContext::getInputContext() on the 2026-07-28 leg — where elicit() threw outright. Portable handlers had to be written in the second shape and carry the fork.

elicit() and elicitUrl() now serve both. Same signature, same return type:

$answer = $context->getClientGateway()->elicit('When?', $schema, key: 'when');

Under 2026-07-28 the gateway's fiber suspension is fulfilled from MRTR state instead of a server-initiated request: answered already → the fiber resumes and the handler runs on; not yet → the ask becomes the input_required result and the fiber is abandoned (unwound, so finally runs), and the same call returns the answer when the client re-sends it. New Mcp\Server\Stateless\ElicitationReplay owns the keying, lookup and sealing; ClientGateway gains only the optional $key.

What it costs:

• One handler entry per ask, so side effects belong after the last question.
• One round trip per ask. InputRequiredResult stays the explicit form for batching several asks into one round, carrying your own state, or asking for anything else.
• Earlier rounds' answers travel in the requestState — the client only echoes the round it just answered — so a handler asking twice or more needs Builder::setRequestState(). Without it: a logged internal error, not silent data loss. Single-ask handlers need nothing.
• key: names an ask across rounds; unnamed asks are keyed by position.

sample() and listRoots() still raise a LogicException there — that revision removed them outright, and the message now says so instead of pointing at InputRequiredResult.

examples/server/elicitation lost its fork entirely: the private ask() is one elicit() call, and every tool dropped its |InputRequiredResult return type and instanceof branch. The pre-existing DualEraElicitationTest drives that example from both eras and passes untouched.

Tests: ElicitationReplayTest plus round-trip, three-round-with-state, missing-key, malformed-answer, undeclared-capability and mid-stream cases in StatelessProtocolTest.