pkg/util: harden TestRandomSuffixGenerator against birthday collisions

The previous test asserted that no two adjacent samples from a
~10 000-element space were equal across 100 iterations. The birthday
math gives that adjacency check a ~1 % flake rate per run; with the
new gitea.lan CI volume that turned into observable random failures
(go-unit #51 on feature/enable-actions-cache hit "Should not be:
'6635'").

Replace adjacency with a distinctness floor over a wider 200-sample
draw. A stuck generator (single value) lands at 1 unique; a
256-element range lands at ~196; the natural full-range generator
lands at ~198. A floor of 150 catches the failure modes the test was
actually written to guard against and never trips on legitimate
randomness.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/util/string_test.go

@@ -282,12 +282,24 @@ func TestAppendRandomSuffixGenerator(t *testing.T) {
 }
 
 func TestRandomSuffixGenerator(t *testing.T) {
-	var last string
-	for range 100 {
+	// The generator draws from a ~10 000 element space (Intn(9999)
+	// formatted as four digits). Comparing each sample against the
+	// previous one with NotEqual flaked ~1 % per 100-iteration run on
+	// natural collisions. Count unique values instead — if the
+	// generator ever gets stuck on a tiny range we still catch it,
+	// without depending on the birthday paradox not firing today.
+	const samples = 200
+	seen := make(map[string]struct{}, samples)
+	for range samples {
 		s := util.RandomSuffixGenerator()
 		assert.Len(t, s, 4)
-		assert.NotEqual(t, last, s)
 		assert.True(t, strings.ContainsFunc(s, func(r rune) bool { return r >= '0' && r <= '9' }))
-		last = s
+		seen[s] = struct{}{}
 	}
+	// In 200 draws from ~10 000 the expected number of unique values
+	// is ~198; a stuck generator (single value) would land at 1, a
+	// 256-value range at ~196. 150 is well above the floor either
+	// way and well below the expected mean.
+	assert.GreaterOrEqual(t, len(seen), 150,
+		"RandomSuffixGenerator drew from too small a range over %d samples", samples)
 }