Fix MTP runner to report error when given .dll for non-Core workloads

[Copilot]

The MTP (Microsoft Testing Platform) runner in Arcade was incorrectly handling .dll files for non-Core (.NET Framework) workloads by executing them directly via shell execute. This could lead to unintended behavior, such as opening the .dll with associated programs (like ILSpy) instead of running tests.

Problem

When TestRuntime is not 'Core', the _TestRunner property was set to $(_TestAssembly) (line 31 in Microsoft.Testing.Platform.targets). If the assembly was a .dll file, it would be executed directly, causing the system to launch whatever program is associated with .dll files.

Solution

Added validation to check that non-Core workloads use executable (.exe) files instead of .dll files:

<!-- Validate that non-Core workloads use executable files, not DLL files -->
<Error Condition="'$(_TestRuntime)' != 'Core' and !$(_TestAssembly.EndsWith('.exe', StringComparison.OrdinalIgnoreCase))" 
       Text="MTP runner for non-Core workloads requires an executable (.exe) file, but was given: '$(_TestAssembly)'. Please set &lt;OutputType&gt;Exe&lt;/OutputType&gt; in your project file." />

Features

• Clear error message: Provides actionable guidance to set <OutputType>Exe</OutputType> in the project file
• Case-insensitive: Accepts both .exe and .EXE extensions
• Non-breaking: Only affects non-Core workloads; Core workloads continue to work with .dll files as expected

Testing

Verified the fix with multiple test scenarios:

• ✅ .dll files for non-Core workloads produce clear error message
• ✅ .exe files (both .exe and .EXE) for non-Core workloads pass validation
• ✅ .dll files for Core workloads continue to work (not affected)

Fixes #15884.

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[nohwnd]

@copilot LGTM, publish it.

[Youssef1313]

LGTM. There is a small follow-up I'd like to do but I'm afraid to write here and Copilot tries to pick it up and tries to do the change in this PR 😄

Edit: #15884 (comment)