Add JSON response mode to Streamable HTTP server

[lntutor]

Summary

• add opt-in EnableJsonResponse configuration for ASP.NET Core and StreamableHttpServerTransport
• return the final correlated JSON-RPC response directly as application/json without SSE framing
• omit intermediate request-related messages and POST event-store writes in JSON mode
• preserve default SSE behavior, empty 202 Accepted notification responses, and standalone GET SSE streams
• reject polling when JSON responses are enabled and document the transport tradeoffs

Testing

• dotnet test tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj --filter FullyQualifiedName~JsonResponseModeTests --no-restore (15 passed across net8.0, net9.0, and net10.0)
• dotnet test tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj -f net10.0 --no-restore --filter "(Execution!=Manual)" (512 passed)
• dotnet test ModelContextProtocol.slnx --configuration Release --no-build --filter "(Execution!=Manual)" (8,454 passed)
• dotnet build ModelContextProtocol.slnx --configuration Release (36 projects, 0 warnings)
• dotnet format ModelContextProtocol.slnx --no-restore --verify-no-changes --include ...
• dotnet docfx docs/docfx.json --warningsAsErrors true (0 warnings)

Fixes #545

[halter73]

The EnableJsonResponse name and ordinary POST behavior match other SDKs, but those SDKs still permit SSE for cases such as subscriptions/listen or standalone GET. Is that the contract we want, or should this be an explicit mode API like ResponseMode = HttpResponseMode.JsonOnly?

For deployments where intermediaries buffer streaming responses, JsonOnly should mean no SSE anywhere. Ordinary POSTs return JSON, GET returns 405, subscriptions/listen returns a correlated MethodNotFound, and EnableLegacySse is incompatible.

We could add a JsonPreferred mode if the hybrid behavior is valuable, but I suspect disabling SSE entirely is the primary use case. Am I off base here?