Add an optional MCP multi round-trip confirmation flow

[jongalloway]

Summary

Add a focused Multi Round-Trip Request (MRTR) exercise using MCP C# SDK 2.x so attendees can learn how a stateless tool requests user input before a consequential action.

Follow-up to #616 and complementary to the optional stateless HTTP lab. MRTR was deliberately deferred during the 2.2.0 migration because the current read-only tools do not justify the interaction complexity.

Proposed scenario

Extend the optional Part 6 business server with a consequential operation such as cancelling an order or approving a return. The tool should request confirmation and an optional reason before making the simulated change.

Required behavior

• Input supplied up front succeeds without a round trip.
• InputRequiredException requests elicitation with opaque requestState.
• Accepted input completes the action.
• Declined or cancelled input leaves state unchanged.
• A down-level stateless client receives actionable guidance and can resubmit required input directly.
• A compatible SDK client handles the input_required response and retry automatically.

Acceptance criteria

• The scenario uses the 2026-07-28 MRTR APIs, not deprecated server-initiated elicitation as the default.
• Accepted, declined, cancelled, up-front, and down-level fallback paths are deterministic and tested.
• requestState contains no secret or trusted client-supplied state.
• The client path is runnable without relying only on Copilot UI behavior.
• The lesson explains why MRTR replaces session-bound elicitation, sampling, and roots for stateless servers.
• The exercise fits optional workshop pacing or is clearly marked as follow-on material.
• Release builds are warning-free and documentation lint/link checks pass.

References

• MCP C# SDK 2.0 announcement
• MCP specification revision 2026-07-28