Avoid reading Configuration in Directory.Build.props

[lntutor]

Summary

• keep configuration-independent artifact base paths in Directory.Build.props
• move package and test-result paths to Directory.Build.targets, after Configuration is initialized
• import the root targets from src/Directory.Build.targets and require SDK 10.0.101 or newer

Closes #995

Test plan

• dotnet build ModelContextProtocol.slnx --configuration Debug --no-incremental
• dotnet build ModelContextProtocol.slnx --configuration Release --no-incremental
• dotnet test ModelContextProtocol.slnx --configuration Debug --no-build --no-restore --filter "(Execution!=Manual)" (8,862 passed)
• dotnet pack ModelContextProtocol.slnx --configuration Release --no-build --no-restore
• verify Debug and Release MSBuild property evaluation for output, intermediate, package, and test-result paths
• direct analyzer test run writes TRX output under artifacts/TestResults/Debug

[lntutor]

The first Windows Debug run had two timing-sensitive ASP.NET Core protocol tests fail on one target framework; the same test assembly passed on the other Windows target frameworks, Windows Release passed, and all Linux/macOS jobs passed. The build output paths were correct. I pushed an empty CI-only commit to rerun the matrix because contributors cannot rerun upstream jobs directly.

[lntutor]

The rerun confirms this is unrelated to the Directory.Build.props change: the Windows Debug build succeeded with zero warnings/errors, and every test assembly reported zero failures (including 2,341/2,341 on net8/net9/net10 and 2,041 passed with expected skips on net472). The job exits 1 only after the blame collector reports that procdump64.exe is missing from PATH. All other matrix jobs are green. This appears to be an upstream Windows runner/blame-collector issue rather than a product or PR failure.