Test that idle host semaphores and .meta files are removed

New tests, failing before the fix: fetches from many hosts, and fetches
that end without a connection, must leave no semaphore in hostSems once
they finish; VariantStorage.Delete must remove the variant's .meta file,
and must succeed when that file is already missing.

Model: opus-5-5

internal/httpfetcher/fetch_internal_test.go

@@ -194,6 +194,14 @@ func semLen(f *HTTPFetcher, host string) int {
 	return len(f.getHostSemaphore(host))
 }
 
+// hostSemCount reports how many hosts have a semaphore in hostSems.
+func hostSemCount(f *HTTPFetcher) int {
+	f.hostSemMu.Lock()
+	defer f.hostSemMu.Unlock()
+
+	return len(f.hostSems)
+}
+
 func TestFetchRedirectToPrivateIPBlocked(t *testing.T) {
 	t.Parallel()