test(pgtype): move interval malformed-text test into interval_test.go

Consolidate TestIntervalScanMalformedTimeReturnsError into the existing
interval_test.go and remove the separate interval_malformed_test.go file.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jackc

pgtype/interval_malformed_test.go

@@ -156,3 +156,15 @@ func TestIntervalTextEncode(t *testing.T) {
 		assert.Equalf(t, tt.result, string(buf), "%d", i)
 	}
 }
+
+// A malformed interval whose time component has an empty hours segment
+// (for example ":05:06") must return an error instead of panicking with an
+// index out of range on timeParts[0][0].
+func TestIntervalScanMalformedTimeReturnsError(t *testing.T) {
+	for _, src := range []string{":05:06", "1 day :30:00"} {
+		var v pgtype.Interval
+		if err := v.Scan(src); err == nil {
+			t.Errorf("Scan(%q): expected error, got nil", src)
+		}
+	}
+}