fix: Detect appends to files kept open by their writer on Windows

[StephKll3c]

On Windows, the polling watcher relied on os.Stat, which reads the directory entry. NTFS does not refresh that entry while another process keeps the file open for writing, which is exactly what SQL Server does with its ERRORLOG. The watcher never saw the file grow, so the tail silently stopped after the initial read (see SekoiaLab/platform#89869, and most likely SekoiaLab/platform#83229 as well, since the fsnotify bump never touched the polling path). The polling loop now stats the file through a handle on Windows, which always returns fresh metadata.

While in there, the polling loop no longer calls util.Fatal on an unexpected stat error, which was killing the whole agent process with os.Exit(1) from a background goroutine. Transient stat errors are now tolerated for a few seconds, then the file is reported as deleted so the tail reopens it instead of dying.