When users run dotnet test with file or directory arguments as positional parameters, they previously received confusing MSBuild or VSTest errors. This PR adds validation to detect these patterns and provide helpful guidance for Microsoft Testing Platform (MTP) users.

Problem

Users often try commands like:

• dotnet test MySolution.sln
• dotnet test MyProject.csproj
• dotnet test path/to/directory
• dotnet test test.dll

These would result in confusing errors like:

• "Solution file error MSB5010: No file format header found"
• "Project file could not be loaded. Data at the root level is invalid"
• "Specify a project or solution file. The current working directory does not contain a project or solution file"

Solution

Added validation in the Microsoft Testing Platform command path (TestingPlatformCommand.cs) to detect file/directory arguments and suggest using the appropriate flags directly (--solution, --project, --directory, --test-modules).

Examples

Before:

$ dotnet test MySolution.sln
MySolution.sln : Solution file error MSB5010: No file format header found.

After (when MTP is enabled):

$ dotnet test MySolution.sln
Solution file 'MySolution.sln' was provided as a positional argument.
Use the --solution flag: dotnet test --solution MySolution.sln

Implementation Details

• Validation only applies when Microsoft Testing Platform is enabled
• Validation triggers for files that actually exist to avoid interfering with MSBuild's normal error handling
• Non-breaking change - existing functionality preserved for VSTest path
• Comprehensive test coverage for all validation scenarios in the MTP command path
• VSTest path (TestCommand.cs) remains unchanged to preserve existing behavior

Fixes #49710.

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