Windows/Linux CI intermittently reddened on t.TempDir auto-cleanup
(RemoveAll: "being used by another process" on Windows, "directory not
empty" on Linux) even though every assertion passed — across the boot
legacy-migration tests and the desktop capabilities tests. #3371 blamed
t.Chdir and fixed one test, but that same test kept failing afterward, so
the real cause is a teardown race, not the cwd.

Two independent fixes:

1. jobs.Manager.Close only cancelled the session context and returned
   without waiting for job goroutines to observe the cancel and exit, so a
   caller tearing down a t.TempDir could race a goroutine still holding a
   file under it. Close now waits on a WaitGroup — shutdown is graceful.

2. Add a robustTempDir test helper (boot + desktop) that retries RemoveAll
   for a short window, absorbing the brief post-Close interval where a
   background resource still holds a file under the dir. A dir that never
   frees is logged, not fatal, so a genuine leak stays visible.