[Client] Add client-side elicitation support

[wachterjohannes]

Summary

The SDK already supports server-initiated elicitation (ClientGateway::elicit()), but the client could not receive elicitation/create requests:

• there was no client request handler for it, and
• ElicitRequest was not registered in MessageFactory, so incoming elicitation/create messages failed to parse (InvalidInputMessageException).

This PR closes that gap, mirroring the existing Sampling pattern one-to-one.

Changes

• MessageFactory — register ElicitRequest::class so elicitation/create parses (the critical fix).
• ElicitationRequestHandler — handles incoming elicitation/create, wraps a user callback, returns Response<ElicitResult> or an Error.
• ElicitationCallbackInterface — the __invoke(ElicitRequest): ElicitResult contract users implement.
• ElicitationException — throw from a callback to forward a specific message to the server; any other throwable returns a generic error (same semantics as SamplingException).
• Unit tests for the handler + a MessageFactory parse test.
• Runnable example examples/client/stdio_elicitation.php against the existing elicitation demo server.
• Docs: docs/client.md Elicitation section + README.md capability entries.
• ROADMAP.md: tick "Implement full support for elicitations" (now complete end-to-end).

Usage

$client = Client::builder()
    ->setCapabilities(new ClientCapabilities(elicitation: true))
    ->addRequestHandler(new ElicitationRequestHandler($myCallback))
    ->build();

Verification

• New unit tests pass (handler accept/decline/exception/generic-error paths + elicitation/create parsing).
• End-to-end: ran examples/client/stdio_elicitation.php against examples/server/elicitation/server.php — book_restaurant round-trips elicited content into a confirmed booking; confirm_action returns the boolean.
• php-cs-fixer clean; no new PHPStan errors introduced.