SIENTIAPDE-1975: align the gateway write path with the Laborious

- GW1: write commands (C2) carry only the server_id; ServerConfigResolver
  resolves the connection (command override, live pooled connection,
  connectors, opc-servers) with a 60 s cache and a 10 s negative cache.
- GW2: write_status keeps the worst outcome (ok < error < session_bad,
  pending/NULL replaced), same rule as the Laborious, built with SQLAlchemy
  Core (no SQL formatting).
- GW3: update_write_status never raises; outcome (updated, kept,
  row_not_found, invalid, error) logged and counted
  (opc_write_status_update_total); runs off the event loop.
- GW4: the reason of a failed write is logged; opc_write_status_by_model
  shows the last outcome; a bad timestamp is a failed write, not an
  exception.
- GW5: the NATS first connect gives up after 5 s (nats-py's defaults block
  the main loop for about 4 minutes while NATS is down); reconnection after
  it is unbounded; idle_sweep restarts a connection closed for good; stop()
  is safe on a closed connection.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/engines/test_write.py

@@ -7,8 +7,17 @@ from asyncua import ua
 import ingestor.metrics as metrics
 from ingestor.engines.write import WriteEngine
 
+SERVER_CFG = {'url': 'opc.tcp://kepserver:49320', 'server_uri': 'urn:kep', 'use_security': True}
+
+
+def make_resolver(cfg=SERVER_CFG):
+    resolver = MagicMock()
+    resolver.resolve = AsyncMock(return_value=cfg)
+    return resolver
+
 
 def make_engine(**kwargs):
+    kwargs.setdefault('servers', make_resolver())
     eng = WriteEngine(
         pool=AsyncMock(),
         sink=AsyncMock(),
@@ -257,3 +266,104 @@ async def test_no_rate_limit_when_min_pause_is_zero():
         await eng.write(base_cmd())
         await eng.write(base_cmd())
         sleep_mock.assert_not_awaited()
+
+
+# --- SIENTIAPDE-1975: server resolution (GW1) and error reporting (GW4) ---
+
+
+@pytest.mark.asyncio
+async def test_write_uses_the_resolved_server_connection():
+    """The C2 command of the Laborious carries only the server_id."""
+    eng = make_engine()
+    eng.pool.get.return_value = make_client()
+
+    await eng.write(base_cmd())
+
+    eng.servers.resolve.assert_awaited_once_with('srv1', base_cmd())
+    eng.pool.get.assert_awaited_once_with('srv1', SERVER_CFG)
+
+
+@pytest.mark.asyncio
+async def test_write_unknown_server_records_error_without_connecting():
+    eng = make_engine(servers=make_resolver(cfg=None))
+
+    result = await eng.write(base_cmd())
+
+    assert result.status == 'error'
+    assert result.opc_status_code == 'server_not_found'
+    eng.pool.get.assert_not_awaited()
+    eng.sink.update_write_status.assert_awaited_once_with(base_cmd(), result)
+
+
+@pytest.mark.asyncio
+async def test_write_server_lookup_failure_records_error():
+    resolver = make_resolver()
+    resolver.resolve.side_effect = RuntimeError('mongo down')
+    eng = make_engine(servers=resolver)
+
+    result = await eng.write(base_cmd())
+
+    assert result.status == 'error'
+    assert result.opc_status_code == 'server_lookup'
+    assert 'mongo down' in result.message
+    eng.pool.get.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_write_without_resolver_uses_the_command_connection():
+    eng = make_engine(servers=None)
+    eng.pool.get.return_value = make_client()
+
+    await eng.write(base_cmd(url='opc.tcp://h:4840', server_uri='urn:h', use_security=True))
+
+    eng.pool.get.assert_awaited_once_with(
+        'srv1', {'url': 'opc.tcp://h:4840', 'use_security': True, 'server_uri': 'urn:h'}
+    )
+
+
+@pytest.mark.asyncio
+async def test_write_without_resolver_or_url_is_server_not_found():
+    eng = make_engine(servers=None)
+
+    result = await eng.write(base_cmd())
+
+    assert result.opc_status_code == 'server_not_found'
+    eng.pool.get.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_write_error_logs_the_reason_and_sets_the_model_status():
+    eng = make_engine()
+    node = make_node()
+    node.write_value.side_effect = RuntimeError('BadWriteNotSupported')
+    eng.pool.get.return_value = make_client(node)
+
+    await eng.write(base_cmd())
+
+    logged = eng.logger.error.call_args.args[0]
+    assert 'RuntimeError' in logged and 'BadWriteNotSupported' in logged
+    assert 'addr=ns=2;i=5' in logged and 'model_id=m1' in logged
+    status_call = next(
+        c
+        for c in eng.emit_metric.await_args_list
+        if c.kwargs['metric_object'] is metrics.OPC_WRITE_STATUS_BY_MODEL
+    )
+    # Same series as a success, so it shows the last outcome.
+    assert status_call.kwargs['value'] == 0
+    assert status_call.kwargs['tags'] == {'pod_id': eng.pod_id, 'model_id': 'm1', 'status': 'ok'}
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('timestamp', [None, 'not-a-timestamp'])
+async def test_write_bad_timestamp_records_error_without_raising(timestamp):
+    eng = make_engine(write_timestamp=True)
+    cmd = base_cmd(timestamp=timestamp)
+    if timestamp is None:
+        del cmd['timestamp']
+
+    result = await eng.write(cmd)
+
+    assert result.status == 'error'
+    assert result.opc_status_code == 'bad_timestamp'
+    eng.pool.get.assert_not_awaited()
+    eng.sink.update_write_status.assert_awaited_once()