Skip flaky FileConfigurationProvider test on .NET Framework (#126874)

> [!NOTE]
> This PR was AI/Copilot-generated.

Fixes #126787

## Problem

The
`ResolveFileProvider_WithMissingParentDirectory_WatchTokenFiresWhenFileCreated`
test introduced in #126411 is flaky on the `net481-windows-Release-x86`
CI leg, causing it to time out and block CI.

## Root cause

`FileSystemWatcher` on .NET Framework drops directory creation events
under I/O contention. The `PendingCreationWatcher` in
`PhysicalFilesWatcher` relies on FSW to detect when a missing root
directory is created. When the FSW event is lost, the change token never
fires and the test hangs.

This was verified by:
1. **Reproducing locally** — running the test in parallel (4-8
concurrent `dotnet test` processes) reproduces the hang on net481 x86
(~3% failure rate).
2. **Isolating to raw FSW** — a minimal test using only
`FileSystemWatcher` (no `PendingCreationWatcher`) also fails under the
same conditions on .NET Framework.
3. **Confirming .NET is unaffected** — 504 runs on net11.0 x86 with 8
concurrent threads produced 0 failures.

All 7 CI failures in the issue report are on the same
`net481-windows-Release-x86-NET481_Release-Windows.10.Amd64.Client.Open`
configuration.

## Changes

- **Skip on .NET Framework**: Changed `[Fact]` to
`[ConditionalFact(typeof(PlatformDetection),
nameof(PlatformDetection.IsNotNetFramework))]` since the test depends on
FSW reliability that .NET Framework cannot guarantee under load.
- **Improved timeout**: Replaced `CancellationTokenSource`-based timeout
with `Task.WaitAsync(TimeSpan)` (via `TaskTimeoutExtensions` polyfill)
and added `TaskCreationOptions.RunContinuationsAsynchronously` —
matching the pattern used by all similar tests in
`PhysicalFilesWatcherTests`.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: svick

src/libraries/Microsoft.Extensions.Configuration.FileExtensions/tests/FileConfigurationProviderTest.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
-using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Configuration.Test;
 using Microsoft.Extensions.FileProviders;
@@ -114,7 +113,8 @@ public void ProviderThrowsDirectoryNotFoundExceptionWhenNotFound(string physical
             Assert.Contains(physicalPath, exception.Message);
         }
 
-        [Fact]
+        // FileSystemWatcher is unreliable under load on .NET Framework, making this test flaky
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotNetFramework))]
         public async Task ResolveFileProvider_WithMissingParentDirectory_WatchTokenFiresWhenFileCreated()
         {
             // Verify the fix for https://github.com/dotnet/runtime/issues/116713:
@@ -150,18 +150,16 @@ public async Task ResolveFileProvider_WithMissingParentDirectory_WatchTokenFires
             Assert.NotNull(token);
 
             // The token should fire only when the target file is created, not when just the directory appears.
-            var tcs = new TaskCompletionSource<bool>();
-            using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(15));
-            cts.Token.Register(() => tcs.TrySetCanceled());
-            token.RegisterChangeCallback(_ => tcs.TrySetResult(true), null);
+            var tcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
+            using var changeCallbackRegistration = token.RegisterChangeCallback(_ => tcs.TrySetResult(true), null);
 
             Directory.CreateDirectory(missingSubDir);
             await Task.Delay(500);
             Assert.False(tcs.Task.IsCompleted, "Token must not fire when only the directory is created.");
 
             File.WriteAllText(configFilePath, "{}");
 
-            Assert.True(await tcs.Task, "Change token did not fire after the target file was created.");
+            await tcs.Task.WaitAsync(TimeSpan.FromSeconds(30));
         }
 
         public class FileInfoImpl : IFileInfo

src/libraries/Microsoft.Extensions.Configuration.FileExtensions/tests/Microsoft.Extensions.Configuration.FileExtensions.Tests.csproj

@@ -8,6 +8,8 @@
   <ItemGroup>
     <Compile Include="$(CommonTestPath)Extensions\ConfigurationRootTest.cs" Link="Common\Extensions\ConfigurationRootTest.cs" />
     <Compile Include="$(CommonTestPath)System\IO\TempDirectory.cs" Link="Common\System\IO\TempDirectory.cs" />
+    <Compile Include="$(CommonTestPath)System\Threading\Tasks\TaskTimeoutExtensions.cs"
+             Link="Common\System\Threading\Tasks\TaskTimeoutExtensions.cs" />
 
     <TrimmerRootDescriptor Include="$(ILLinkDescriptorsPath)ILLink.Descriptors.Castle.xml" />
   </ItemGroup>