Elicitation migration

[g2vinay]

Objective: Rebuild user-consent flows to the RC multi-round model and keep security behavior correct.

Work items:

1. Replace or adapt the current wrapper in core/Microsoft.Mcp.Core/src/Extensions/McpServerElicitationExtensions.cs to the RC client/server API.
2. Update core/Microsoft.Mcp.Core/src/Areas/Server/Commands/ToolLoading/BaseToolLoader.cs so consent checks work with request-scoped multi-round interactions.
3. Ensure sensitive or destructive operations still fail closed when elicitation is unavailable.
4. Decide whether current consent UI text should stay as-is or be revised to reflect the new request/response flow.
5. Add tests for accept, decline, cancel, and continuation behavior.

Acceptance criteria:

• A sensitive tool call can start, pause for consent, resume, and complete under the new protocol.
• No consent request escapes the active request lifecycle.
• Client-not-supported paths still fail safely.