Compose MRTR input requests with task-backed tools

[KirschBluteX]

Summary

• route InputRequiredException from task-backed [McpServerTool] executions through the existing task input-request interceptor
• replay the tool after tasks/update, carrying InputResponses and RequestState into both the typed and JSON-RPC request parameters
• cover elicitation, sampling, and roots requests, including cancellation, input-handler failures, waiter cleanup, and the legacy synchronous MRTR fallback
• document explicit MRTR composition with Tasks

This implements Case 1 only. Case 2, mid-execution promotion through a DeferTaskCreation/PromoteToTaskAsync-style API, remains separate pending its public API and lifecycle design.

Partially addresses #1635

Testing

• dotnet build ModelContextProtocol.slnx --configuration Release --no-restore -m:1 --verbosity minimal (36 projects, 0 warnings, 0 errors)
• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --configuration Release --framework net10.0 --no-build --no-restore --filter "FullyQualifiedName~McpTaskStoreTests|FullyQualifiedName~MrtrServerBackcompatTests|FullyQualifiedName~MrtrInputRequiredExceptionTests|FullyQualifiedName~MrtrReturnedInputRequiredResultNativeTests|FullyQualifiedName~MrtrHandlerLifecycleTests" --verbosity minimal (71 passed)
• dotnet format tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --verify-no-changes --no-restore --include tests/ModelContextProtocol.Tests/Server/McpTaskStoreTests.cs src/ModelContextProtocol.Extensions.Tasks/Server/McpTasksBuilderExtensions.cs --verbosity minimal
• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --configuration Release --framework net10.0 --no-build --no-restore --filter "Execution!=Manual" --verbosity minimal (2361 passed)
• DockerEverythingServerTests.ConnectAndReceiveMessage_EverythingServerWithSse (1 passed)
• DockerEverythingServerTests.Sampling_Sse_EverythingServer (manual external-dependency test) still fails in isolation because tzolov/mcp-everything-server:v1 resolves to the 2024-12-15 image, which exposes sampleLLM rather than the test's expected trigger-sampling-request; pulling the tag again returns the same digest.