Fix delay issue in serving SinglePageApp index.html on Windows

Summary & Motivation

This PR resolves an issue where the backend experienced delays when serving the index.html file of a SinglePageApp. The delay occurred because the backend was waiting for the index.html file to regenerate before serving it, ensuring the file included references to newly generated JavaScript bundles. The logic now checks for updates using the LastWriteTime to confirm when the new file is ready.

The delay primarily affected Windows due to differences in how file timestamps are handled across operating systems. On Windows, the CreationTime doesn't update when a file is modified, unlike on MacOS where both CreationTime and LastWriteTime are updated. The fix ensures consistent behavior across platforms by relying on the LastWriteTime.

Atomic Changes

Fix issue when waiting extensively on generation of SinglePageApp files

Checklist

[x] I have added a Label to the pull-request
[x] I have added tests, and done manual regression tests
[x] I have updated the documentation, if necessary

platformplatform / PlatformPlatform