Flaky OuterLoop test: FileSystemWatcher_File_Changed_NotAffectEachOther on macOS

## Build Information
Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1512856
Build error leg or test failing: System.IO.Tests.FileSystemWatcher_Multiple_Test.FileSystemWatcher_File_Changed_NotAffectEachOther
Pull request:

## Error Message

**DO NOT USE JSON BELOW IF THIS IS A BUILD BREAK** otherwise build analysis will allow pull requests to merge that break the build worse. For a build break, do not use this issue form. Make a regular new issue.

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Build%20Analysis/KnownIssueJsonStepByStep.md).

```json
{
"ErrorMessage": "System.IO.Tests.FileSystemWatcher_Multiple_Test.FileSystemWatcher_File_Changed_NotAffectEachOther [FAIL]",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

```
System.IO.Tests.FileSystemWatcher_Multiple_Test.FileSystemWatcher_File_Changed_NotAffectEachOther [FAIL]
Assert.False() Failure
Expected: False
Actual: True
Stack Trace:
/_/src/libraries/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.MultipleWatchers.cs(257,0): at System.IO.Tests.FileSystemWatcher_Multiple_Test.FileSystemWatcher_File_Changed_NotAffectEachOther()
/_/src/coreclr/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.CoreCLR.cs(37,0): at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
/_/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.cs(57,0): at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
```

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1512856
**Error message validated:** `[System.IO.Tests.FileSystemWatcher_Multiple_Test.FileSystemWatcher_File_Changed_NotAffectEachOther [FAIL]`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 7/17/2026 11:04:07 AM UTC

### Report
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|0|0|

Start with System.IO.FileSystem.Watcher/tests/FileSystemWatcher.MultipleWatchers.cs at line 257 and inspect FileSystemWatcher_File_Changed_NotAffectEachOther. Use the linked build result to confirm the macOS failure and investigate why the assertion reports an unexpected event. Done means the test passes reliably on macOS without recurring flakes.

Written by the indexing model from the issue text.