test(purgatory): tolerate scheduler delay in activity fixture

The activity fixture emitted every 20 ms but allowed only 50 ms of inactivity. Under the exact parallel CI command, CPU-heavy unit tests can delay pipe-reader scheduling beyond that margin and terminate a subprocess that continues producing output.

Raise only the fixture inactivity limit to 500 ms while retaining its two-second overall deadline and exact output assertion. Production policy and the separate whole-process-group inactivity test are unchanged.

Validated with the focused fetch-helper test.

src/purgatory/sync/context.rs

@@ -1331,7 +1331,7 @@ mod fetch_helper_tests {
                 "fetch_batch",
                 GitFetchRole::Primary,
                 GitProcessPolicy {
-                    inactivity_limit: Duration::from_millis(50),
+                    inactivity_limit: Duration::from_millis(500),
                     termination_grace: Duration::from_millis(50),
                 },
             ),