Add more info for StdErrAfter assertion in host tests

[elinor-fung]

The assertion only ended up printing stderr, but it would be useful to have all the information about the command that was run (the exit code, stdout). Without it, we have no insight into issues like #118170, especially since it does not seem to repro easily (first time we've seen it, hasn't been hit again).

Resolves #118170 - while this wouldn't actually fix whatever happened there, it will give us insight into any similar issues that happen again.

cc @dotnet/appmodel @AaronRobinsonMSFT

[Copilot]

Pull Request Overview

This PR enhances the test infrastructure for host tests by improving the diagnostics information provided when StdErr assertions fail. Instead of only showing stderr output, it now provides comprehensive command execution details including file name, arguments, environment variables, exit code, and both stdout and stderr.

• Moves GetDiagnosticsInfo() method from CommandResultAssertions to CommandResult class for better encapsulation
• Updates assertion error messages to use the enhanced diagnostics information
• Filters environment variables to show DOTNET_ prefixed variables instead of just DOTNET_ROOT

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/installer/tests/TestUtils/CommandResult.cs	Adds GetDiagnosticsInfo() method to provide comprehensive command execution details
src/installer/tests/TestUtils/Assertions/CommandResultExtensions.cs	Updates StdErr assertion error message to use new diagnostics info
src/installer/tests/TestUtils/Assertions/CommandResultAssertions.cs	Removes GetDiagnosticsInfo() method and delegates to CommandResult implementation

[dotnet-policy-service]

Tagging subscribers to this area: @vitek-karas, @agocke, @VSadov
See info in area-owners.md if you want to be subscribed.