mcp: URLElicitationRequiredError disagrees with Elicit and ParseURLElicitationRequiredError #1500

Problem

URLElicitationRequiredError treats an empty Mode as "form" and panics, while ServerSession.Elicit infers "url" from URL/ElicitationID (inferElicitMode). It also accepts elicitations with no URL or ElicitationID, and an empty list, so a server can build a -32042 that ParseURLElicitationRequiredError (#1495) rejects. This is the open point from #1495.

Change

• Infer the mode as Elicit does and marshal the inferred copies, so the wire always carries "mode": "url".
• Return a plain (non-*jsonrpc.Error) error for any input a client could not act on: an empty list, a nil elicitation, a non-URL mode, or a missing URL/ElicitationID. No panics: nothing in the SDK recovers a handler panic, so one takes the server process down, and these values are often computed at runtime. Relaxing the current non-URL-mode panic to an error breaks no caller.
• Share one validation helper between the builder and ParseURLElicitationRequiredError, so the two cannot drift again.

This replaces the tests that expect a panic for an empty or "form" mode. Out of scope: Elicit itself does not check for a URL or elicitation ID in URL mode.

I'd like to send the PR for this; I have it ready.