Fix: Fixed issue where search continued running after navigating away or closing a tab

[workbysaran]

Resolved / Related Issues
Fixed an issue where search queries kept running in the background even after the user cleared the search, navigated to a different folder, or closed the tab, causing sustained CPU usage.

Closes #18393

Steps used to test these changes

1. Open Files and navigate to a large drive
2. Open the search bar and type a query slowly to confirm suggestion searches cancel per keystroke and only fire after a ~200ms pause
3. Type a full query and press Enter and confirm search results load correctly
4. While results are loading, navigate to a different folder and confirm the search stops promptly

Note: A brief CPU in Files.exe after cancellation is expected due to Windows Search runs as an in-process COM server and its native thread pool work drains briefly after the .NET task unblocks. Please find the profiler screenshot attached

[Josh65-2201]

I'm not certain the search is stopping, Files shows that it's not loading anything, but CPU usage goes from 0.1% when idle to around 10% start of search and stays there for about 10 seconds before decreasing only a lit bit around 5-8% when backing out to the home page or empty folder. Although this is much less than 4.1.5 staying around 25-30% after backing out

Search results are shown same as 4.1.5