[Client] Add client-side elicitation support (#371)

* [Client] Add client-side elicitation support

The SDK could send elicitation/create requests from a server tool via
ClientGateway::elicit(), but the client could not receive them: there was
no request handler, and ElicitRequest was not registered in MessageFactory,
so incoming elicitation/create messages failed to parse.

This adds the client half, mirroring the existing Sampling pattern:

- Register ElicitRequest in MessageFactory so elicitation/create parses.
- ElicitationRequestHandler wraps a user callback and returns an
  ElicitResult (accept/decline/cancel) or an Error.
- ElicitationCallbackInterface defines the callback contract.
- ElicitationException forwards a specific message to the server; any
  other throwable returns a generic error.

Includes unit tests, a runnable client example against the elicitation
demo server, and client docs/README updates.

* [Client] Simplify elicitation example labelFor() per review

Type the parameter as AbstractSchemaDefinition and read ->title directly
instead of matching on each concrete definition type. title is a required
non-null string on the base class, so the match and the $name fallback were
redundant.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

ROADMAP.md

@@ -5,7 +5,7 @@ This roadmap is a living document that outlines the planned features and improve
 ## Goals for the First Major Release
 
 - **Server**
-- [ ] Implement full support for elicitations
+- [x] Implement full support for elicitations
 - [ ] Implement OAuth2 authentication for server
 - **Client**
 - [x] Implement client-side support