Add elicitation and sampling support

[rdwj]

Context

FastMCP 3.x provides elicitation (requesting user input from the client) and sampling (requesting LLM completions from the client) via the Context object. These are advanced MCP features that mcp-test-mcp should be able to test on target servers.

Scope

• Add tools to test elicitation capabilities on target servers
• Add tools to test sampling/LLM completion requests from target servers
• Test that proxy forwarding of these features works correctly

Notes

This is planned for a future release after the 3.x upgrade stabilizes. Filed to track the intent.