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

[lntutor]

Rebased onto current main and resolved the transport conflicts by preserving both JSON-only responses and the July 2026 HTTP status mapping. Added a regression proving an unknown July 2026 method returns HTTP 404 with an application/json JSON-RPC error in JSON response mode; it fails without the response-start callback and passes with the merged implementation across .NET 8, 9, and 10. The complete ModelContextProtocol.AspNetCore.Tests project also passes locally.

[lntutor]

Fresh verification on cb711d3 is fully green: Ubuntu, macOS, and Windows Debug/Release build-and-test jobs all passed; coverage publication, CodeQL, and markdown-link-check also passed. The prior Windows Debug failure and link 502 did not recur. Ready for review.