fix(lint): apply De Morgan's law to hex-validation guards (QF1001)

Merging origin/master into the lint branch pulled in two test files
(mongo/k8s_test.go, postgres/k8s_helpers_test.go) whose hex-character
guards trip staticcheck QF1001 under CI's golangci-lint `latest`
(newer than the locally pinned 2.11.4). Rewrite

    if !((c >= '0' && c <= '9') || (c >= 'a' && c <= 'f'))
into the equivalent
    if (c < '0' || c > '9') && (c < 'a' || c > 'f')

Pure boolean equivalence — both packages' tests still pass. Also merges
master so the local lint surface matches the pull/16/merge commit CI lints.

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

Author: Manas Srivastava

internal/backend/mongo/k8s_test.go

@@ -117,7 +117,7 @@ func TestMongoK8sRandHex_LengthAndUniqueness(t *testing.T) {
 	}
 	// Hex-only characters.
 	for _, c := range a {
-		if !((c >= '0' && c <= '9') || (c >= 'a' && c <= 'f')) {
+		if (c < '0' || c > '9') && (c < 'a' || c > 'f') {
 			t.Errorf("non-hex char %q in %q", c, a)
 		}
 	}

internal/backend/postgres/k8s_helpers_test.go

@@ -85,7 +85,7 @@ func TestK8sRandHex(t *testing.T) {
 	}
 	// Must be valid hex.
 	for _, c := range s {
-		if !((c >= '0' && c <= '9') || (c >= 'a' && c <= 'f')) {
+		if (c < '0' || c > '9') && (c < 'a' || c > 'f') {
 			t.Errorf("k8sRandHex returned non-hex char %q", c)
 			break
 		}