T5 - MRTR (multi round trip requests) types

Target branch: 2026-07-28-protocol-support

• InputRequest union (sampling/createMessage, roots/list, elicitation/create embedded shapes with method and
  params) and InputResponse union (CreateMessageResult, ListRootsResult, ElicitResult).
• InputRequiredResult(inputRequests, requestState, resultType, _meta).
• inputResponses and requestState on CallToolParams, GetPromptParams, ReadResourceParams.
• ElicitParams becomes a union of Form(message, requestedSchema, mode) and Url(message, url, elicitationId).
  elicitationId stays optional for 2025-11-25 servers.
• Schema test cases against both schema files where the type exists in both.

Spec:

• MRTR - InputRequests
• MRTR - InputResponses
• MRTR - InputRequiredResult
• Tools - Input Required Tool Results
• Elicitation - URL Mode Elicitation Requests
• Schema - InputRequiredResult
• Schema - InputResponseRequestParams

Conformance scenarios that must pass after this task: none.

Docs: no.

Start on target branch 2026-07-28-protocol-support by locating the existing InputRequest, InputResponse, CallToolParams, GetPromptParams, ReadResourceParams, and ElicitParams definitions, then compare them with the linked MRTR and elicitation specifications. Update the protocol types and add schema test cases in both schema files where applicable; done means the requested unions, fields, and optional compatibility behavior are represented and tests pass.