test(#430): fix oversized-frame test for the 176-byte ceiling

The 172->176 ceiling raise made the existing "drops oversized frames and
resyncs" test's 173-byte frame a valid length, so it buffered instead of
dropping (it was red). Derive the over-max length from the ceiling so it stays
a genuine oversized frame and still exercises drop-and-resync, confirming the
higher ceiling doesn't weaken corrupt-frame recovery. Full suite: 643 green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/services/usb_serial_frame_codec_test.dart

@@ -102,11 +102,16 @@ void main() {
     () {
       final decoder = UsbSerialFrameDecoder();
 
+      // A frame claiming a payload above the max must be dropped, and the
+      // decoder must resync to the next valid frame. Length is derived from the
+      // ceiling so this stays a genuine over-max after it was raised to 176
+      // (#430) — a hard-coded 173 is now a valid length.
+      final oversized = usbSerialMaxPayloadLength + 1;
       final packets = decoder.ingest(
         Uint8List.fromList(<int>[
           usbSerialRxFrameStart,
-          0xAD,
-          0x00,
+          oversized & 0xff,
+          (oversized >> 8) & 0xff,
           0x99,
           usbSerialRxFrameStart,
           0x01,