Fix PubSub DataSetWriter sequence number uint16 overflow

[Lidang-Jiang]

Summary

• Fix sequence number overflow in DataSetWriter.generate_uadp_dataset() that crashes the PubSub writer after 65535 messages
• Closes OPC UA PubSub dataset header sequence number uint16 overflow #1953

Changes

• asyncua/pubsub/writer.py: Wrap sequence number with modulo arithmetic: self._seq_no = (self._seq_no % 65535) + 1
• tests/test_pubsub.py: Add regression test verifying sequence number wraps from 65535 back to 1

Root Cause

self._seq_no += 1 increments without bound. When UInt16(self._seq_no) is serialized via struct.pack('<H', value) with a value > 65535, it raises struct.error. At 30Hz publishing frequency, this crashes the writer after ~36 minutes.

Fix

Use (self._seq_no % 65535) + 1 so the sequence number cycles within [1, 65535], matching the valid UInt16 range.

Before (overflow demonstration)

=== Demonstrating the overflow bug (before fix) ===

UInt16 sequence number increments without bound:
  seq_no = 65535 -> struct.pack OK
  seq_no = 65536 -> struct.pack FAIL: 'H' format requires 0 <= number <= 65535
  seq_no = 65537 -> struct.pack FAIL: 'H' format requires 0 <= number <= 65535
  seq_no = 65538 -> struct.pack FAIL: 'H' format requires 0 <= number <= 65535

At 30Hz publishing frequency, this overflow occurs after:
  65535 / 30 = 2184 seconds = ~36.4 minutes

After (fix verified)

=== After fix: sequence number wraps correctly ===

Sequence number behavior around boundary:
  seq_no = 65534 -> struct.pack OK (bytes: feff)
  seq_no = 65535 -> struct.pack OK (bytes: ffff)
  seq_no = 1 -> struct.pack OK (bytes: 0100)
  seq_no = 2 -> struct.pack OK (bytes: 0200)
  seq_no = 3 -> struct.pack OK (bytes: 0300)

Simulating 200,000 messages (no overflow):
  All 200,000 iterations completed without error. Final seq_no=3395

At 30Hz, 200,000 messages = 111 minutes of continuous operation

Test output

$ pytest tests/test_pubsub.py -v

tests/test_pubsub.py::test_uadp_basic PASSED                             [ 16%]
tests/test_pubsub.py::test_connection PASSED                             [ 33%]
tests/test_pubsub.py::test_full_simple PASSED                            [ 50%]
tests/test_pubsub.py::test_datasource_and_subscribed_dataset PASSED      [ 66%]
tests/test_pubsub.py::test_dataset_writer_seq_no_wraps_at_uint16_max PASSED [ 83%]
tests/test_pubsub.py::test_load_save_ua_binary_publisher PASSED          [100%]

============================== 6 passed in 8.41s ===============================

Test plan

• Sequence number wraps from 65535 to 1 (not 0)
• Normal incrementing behavior preserved for values < 65535
• Existing PubSub tests pass (no regression)