Fix OSConditionAttribute to detect OS via reflection for .NET Framework on Mono

[Copilot]

OSConditionAttribute assumed .NET Framework always means Windows, causing [OSCondition(OperatingSystems.Windows)] tests to run on Linux/Mac when using Mono.

Changes

• Use RuntimeInformation.IsOSPlatform via reflection for NETFRAMEWORK builds

  • Loads RuntimeInformation from System.Runtime.InteropServices.RuntimeInformation assembly or mscorlib (Mono)
  • Falls back to assuming Windows if API unavailable (older .NET Framework without the API)

• Cache detected OS at class load time to avoid repeated reflection overhead

• Add exception handling for reflection invocations

// Before: Always assumed Windows for .NET Framework
#if NET462
    => (_operatingSystems & OperatingSystems.Windows) != 0;

// After: Detects actual OS via reflection, falls back to Windows if unavailable
#if NETFRAMEWORK
    {
        get
        {
            OperatingSystems currentOS = s_detectedOS ?? OperatingSystems.Windows;
            return (_operatingSystems & currentOS) != 0;
        }
    }

Fixes #6615

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>OSConditionAttribute include windows doesn't work for netfx Linux</issue_title>
<issue_description>## Describe the bug

When running a test marked with [OSCondition(OperatingSystems.Windows)] with netfx on Linux or Mac, the test will be run via mono and the attribute won't exclude the test.

The attribute currently assumes netfx to be Windows only which is not always right.</issue_description>

<agent_instructions>Comment from jkotas should be followed carefully</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@Youssef1313 @jkotas Is there a good way to detect the OS when compiling for .NET Framework and running with `mono`?

What we have today is:

testfx/src/TestFramework/TestFramework/Attributes/TestMethod/OSConditionAttribute.cs

Lines 53 to 55 in eaaf9f5

	#if NET462
	// On .NET Framework, we are sure we are running on Windows.
	=> (_operatingSystems & OperatingSystems.Windows) != 0;

</comment_new>

• Fixes OSConditionAttribute include windows doesn't work for netfx Linux #6615

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