[Repo Assist] test(tray): add WindowsFactAttribute; skip DPAPI test on non-Windows

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🤖 This is an automated pull request from Repo Assist.

Summary

Adds WindowsFactAttribute — a custom xUnit FactAttribute that automatically skips the test on non-Windows platforms — and applies it to the one Tray test that calls the Windows Data Protection API (DPAPI).

Problem

SettingsManager_ProtectsElevenLabsApiKeyForStorage calls SettingsManager.ProtectSettingSecret, which intentionally throws PlatformNotSupportedException on non-Windows:

if (!OperatingSystem.IsWindows())
    throw new PlatformNotSupportedException("Windows Data Protection API is required...");

The test had no platform guard, so it caused a spurious failure (not a skip) on every Linux run:

Failed OpenClaw.Tray.Tests.SettingsRoundTripTests.SettingsManager_ProtectsElevenLabsApiKeyForStorage
  System.PlatformNotSupportedException: Windows Data Protection API is required for protected settings secrets.

Fix

Adds WindowsFactAttribute following the exact same pattern as the existing IntegrationFactAttribute:

public sealed class WindowsFactAttribute : FactAttribute
{
    public WindowsFactAttribute()
    {
        if (!OperatingSystem.IsWindows())
            Skip = "Windows-only: requires a Windows platform API.";
    }
}

Replaces [Fact] with [WindowsFact] on the DPAPI test. On Windows CI (windows-latest), the test runs and asserts fully. On Linux the test is skipped rather than failing.

Related

This is a companion to #(PR for canvas fix) which fixed the two pre-existing failures in OpenClaw.Shared.Tests (also Linux cross-platform issues). Together these make the full test suite clean on Linux.

Test Status

Suite	Before	After
OpenClaw.Shared.Tests	1152 passed / 0 failed / 20 skipped	✅ unchanged
OpenClaw.Tray.Tests	406 passed / 1 failed / 0 skipped	✅ 406 passed / 0 failed / 1 skipped

./build.ps1 was not run (Linux environment). This change only touches test infrastructure — no production source files modified.

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