Add [SupportedOSPlatformGuard] to OperatingSystem polyfill methods

[Copilot]

The polyfilled OperatingSystem.Is*() methods were missing the [SupportedOSPlatformGuard] attributes present on their native .NET 6+ counterparts. Without them, CA1416 doesn't recognize these methods as platform guards on older TFMs, producing spurious warnings for correctly-guarded code:

if (OperatingSystem.IsWindows())
{
    SomeWindowsOnlyApi(); // CA1416 fires here, even though it's guarded
}

• PolyShim/Net50/OperatingSystem.cs — added [SupportedOSPlatformGuard("…")] to each polyfilled method:
  • IsFreeBSD() → "freebsd"
  • IsLinux() → "linux"
  • IsMacOS() → "macos"
  • IsWindows() → "windows"
  • IsWindowsVersionAtLeast(…) → "windows"

SupportedOSPlatformGuardAttribute is already polyfilled in PolyShim/Net60/SupportedOSPlatformGuardAttribute.cs, so no new dependency is introduced.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.70%. Comparing base (15d4ad5) to head (f6ba876).
⚠️ Report is 1 commits behind head on prime.

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #123      +/-   ##
==========================================
+ Coverage   90.66%   90.70%   +0.04%     
==========================================
  Files         285      285              
  Lines        4959     4959              
  Branches      404      404              
==========================================
+ Hits         4496     4498       +2     
+ Misses        371      369       -2     
  Partials       92       92              

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[Copilot]

Pull request overview

This PR updates the OperatingSystem.Is*() polyfills to match the platform-guard annotations present on modern .NET, allowing analyzers (e.g., CA1416) to recognize these methods as valid OS guards on older TFMs.

Changes:

• Added System.Runtime.Versioning import to the Net50 OperatingSystem polyfill file.
• Annotated IsFreeBSD(), IsLinux(), IsMacOS(), IsWindows(), and IsWindowsVersionAtLeast(...) with [SupportedOSPlatformGuard("…")].

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