Document Streamable HTTP integration testing

[luisangelrod]

Summary

• document how to integration test an ASP.NET Core MCP server with WebApplicationFactory
• pass the factory-owned HttpClient to HttpClientTransport in Streamable HTTP mode
• add an executable sample server and end-to-end echo tool test
• include the sample projects in the solution so the documentation is build verified

Validation

• dotnet build -c Release
• dotnet docfx docs/docfx.json --warningsAsErrors true
• dotnet test docs/concepts/transports/samples/integration-testing/tests/IntegrationTestingMcpServer.Tests.csproj -c Release --no-build --no-restore (1 passed)

The full test suite was also attempted locally. Unrelated environment dependencies involving the local OAuth HTTPS test service and missing npm/Node conformance packages prevented a clean full-suite completion.

Closes #1781

Disclosure: Drafted with AI assistance.