Fix ANCM app_offline.htm notification match using QueryCCH

[aw0lid]

Fix ANCM app_offline.htm notification match using QueryCCH

Description

Fixes an issue in file watcher where FILE_WATCHER::HandleChangeCompletion compared the buffer capacity (QuerySizeCCH()) instead of the actual string length (QueryCCH()) against the notified filename length. This made the notification-driven app_offline.htm match path unreachable, causing the application to rely solely on the fallback existence check.

Changes

• Replaced _strFileName.QuerySizeCCH() with _strFileName.QueryCCH() in filewatcher.cpp.
• Added a test seam (TEST_SetNotificationBuffer) in filewatcher.h under ANCM_TESTING.
• Added native unit tests covering exact matches and prefix rejections in filewatcher_tests.cpp.
• Enabled ANCM_TESTING in CommonLibTests.vcxproj.

Fixes #68283

[dotnet-policy-service]

Thanks for your PR, @aw0lid. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.