dotnet build outputs compilation errors to stdout, not stderr.
The previous code only captured stderr, resulting in empty error messages
when C# samples failed to build.

Changes:

• test_hosted_sample.py: Capture both stdout and stderr in build error
  message
• test-hosted-agents.yml: Print error details from result.json before
  exit 1 in the "Fail if test failed" step

This ensures actual compilation errors (e.g., CS0535 missing interface
implementation) are visible in CI logs instead of just "exit 1".