OSOE-1308: Default .NET test workflows to Microsoft Testing Platform

[Piedone]

Default the modern .NET test workflows to Microsoft Testing Platform and generate GitHub Actions summaries with the native reporter. Use evaluated MSBuild properties to find test applications and process exit codes to determine success, preserving filtered solution runs, test output, TRX artifacts, diagnostics, and hang dump collection. Retain an explicit VSTest option for legacy consumers; msbuild-and-test continues to select it.

Part of Lombiq/Testing-Toolbox#92. Coordinated OSOCE integration: Lombiq/Open-Source-Orchard-Core-Extensions#1335.

This is a breaking change requiring .NET SDK 10+, the global.json MTP runner selection, and reporting extensions in each test project. Docs/MicrosoftTestingPlatform.md describes migration and the v6.0.0 release step. Internal action references use issue/OSOE-1308 during review and must return to dev before merge; publish v6.0.0 through the existing release-branch workflow after integration.

Validation: the local regression fixture passes filtering, theory discovery, passing-test output, native summaries, TRX reporting, diagnostics, empty selections, and failure exit-code checks. PowerShell analysis and YAML validation pass. Added Windows/Ubuntu CI regression coverage. The fixture explicitly clears the expected negative scenario's exit status before returning to the CI shell.

CI verification: the MTP regression suite passes on Ubuntu and Windows; spelling, YAML, branch-reference, and PR validation checks pass.

[sarahelsaig]

Didn't we reject using class primary constructors (as opposed to record primary constructors) in the past? (see)

[sarahelsaig]

This data is only used in a ``foreach` so it doesn't actually need to be a dictionary:

Suggested change

	$savedEnvironment = @{}
	foreach ($name in @('PATH', 'GITHUB_ACTIONS', 'GITHUB_OUTPUT', 'GITHUB_STEP_SUMMARY', 'GITHUB_WORKSPACE', 'LGHA_TEST_FAILURE'))
	{
	$savedEnvironment[$name] = [Environment]::GetEnvironmentVariable($name)
	}
	$environmentVariableNames = @('PATH', 'GITHUB_ACTIONS', 'GITHUB_OUTPUT', 'GITHUB_STEP_SUMMARY', 'GITHUB_WORKSPACE', 'LGHA_TEST_FAILURE')
	$savedEnvironment = Get-ChildItem Env: | Where-Object { $PSItem.Name -in $environmentVariableNames }

[sarahelsaig]

Two unrelated parameters packed on the same line.

Suggested change

	'-NoProfile', '-File', "$actionPath/Invoke-SolutionOrProjectTests.ps1"
	'-NoProfile',
	'-File', "$actionPath/Invoke-SolutionOrProjectTests.ps1"

[sarahelsaig]

To me using Env: feels more appropriate for a PowerShell script.

Suggested change

	foreach ($entry in $savedEnvironment.GetEnumerator())
	{
	[Environment]::SetEnvironmentVariable($entry.Key, $entry.Value)
	}
	$savedEnvironment | ForEach-Object { Set-Item ('Env:' + $PSItem.Key) -Value $PSItem.Value }

or formatted

Suggested change

	foreach ($entry in $savedEnvironment.GetEnumerator())
	{
	[Environment]::SetEnvironmentVariable($entry.Key, $entry.Value)
	}
	$savedEnvironment | ForEach-Object {
	Set-Item ('Env:' + $PSItem.Key) -Value $PSItem.Value
	}

[sarahelsaig]

Since MTP is for .NET 10+ only, do we want to do something like this to reduce the breakingness of the change?

$dotnetVersion = [Version](dotnet --version)
if ($dotnetVersion.Major -lt 10)
{
    $switches['TestPlatform'] = 'VSTest'
    Write-GitHub "The test platform is set to `"VSTest`" because you are using .NET version ($dotnetVersion) older than .NET 10. Please set the `"test-platform`" input to `"VSTest`" explicitly, to remove this warning."
}