Allow HttpClient BaseAddress as transport endpoint

[lntutor]

Summary

• allow HttpClientTransportOptions.Endpoint to be omitted when an injected HttpClient has a BaseAddress
• pass the resolved endpoint explicitly through auto-detect, SSE, and Streamable HTTP session transports
• preserve explicit endpoint precedence and support Aspire-style logical base-address schemes handled by custom HTTP handlers

Testing

• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --filter FullyQualifiedName~HttpClientTransportTests
• dotnet build tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --no-restore

Fixes #515