[chemicL]

Handlers had to check ctx.clientCapabilities() themselves before asking for elicitation, sampling or
roots. Nothing stopped one from sending an input request the client never declared.

• Required capabilities: tools, prompts and resources declare what they can't work without via
  of(..., requiredCapabilities, handler) on McpAsync/SyncTool, McpAsync/SyncPrompt and
  McpAsync/SyncResource. The feature checks this before the handler runs. A client that lacks a
  capability gets -32021, listing only what's missing.
• Optional input: handlers request it only when the client declares the capability, and the
  server enforces that. McpServer rejects any input request for an undeclared capability as an
  internal error, which covers extension methods too. URL-mode elicitation is checked separately
  from form mode.
• Docs: the javadoc on the handler types, InputRequired, McpRequestContext and McpException
  says which strategy to use.