SIENTIAPDE-1974: fix(read): code-review fixes for the slot v2 read path

- config: GatewayConfig.certs_from_env() reads only CERT_BUCKET/CERT_CACHE_DIR;
  IngestorManager uses it, so a malformed write/gRPC env var (GRPC_PORT,
  WRITE_MIN_PAUSE_S, IDLE_CONN_MAX_S) can no longer stop the reads.
- certs: CertProvider raises CertificateNotConfiguredError with an actionable
  message when no platform_certificates document exists; downloads once under
  an asyncio.Lock, caches the paths, writes atomically (tmp + os.replace);
  invalidate() forces a fresh download.
- pool: server_id normalized to str (1 and '1' share a session); warns when a
  get() asks for other url/server_uri/use_security than the pooled client was
  opened with (client reused, the slot config wins); invalidates the cert
  cache when a secured connect fails.
- SubscriptionEngine: tracks nodes per subscription and forgets them on
  unsubscribe (no more cycle alarms / tag count for removed tags), keeping
  addresses still monitored by another slot.
- ServerReader.shutdown(evict) / IngestorManager.remove_server(evict): a
  server that only left this pod's slots keeps the shared connection for
  NATS writes/gRPC; lost servers and config changes still evict.
- IngestorManager warns once per server when a slot still sets
  server_cert_path (server certificate pinning is no longer applied).
- slots: CONNECTION_FIELDS exported and reused by ServerReader.

Accepted, not changed: read metrics label server_name with the server_id
(same as the write path, which only knows the id); a malformed slot is
notified each lease attempt (alerts are deduplicated by notification_id).

Tests: 236 unit tests pass (+18); ruff clean; local e2e against an asyncua
server incl. slot moved (connection kept and reused by the write path) and
lost-server eviction/re-initialization.

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

tests/unit/connection/test_pool.py

@@ -248,3 +248,63 @@ async def test_evict_unknown_server_is_noop():
     await pool.evict('nope')
 
     pool.emit_metric.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_get_normalizes_server_id_to_str():
+    pool = make_pool()
+    fake_client = AsyncMock()
+    with patch('ingestor.connection.pool.Client', return_value=fake_client):
+        c1 = await pool.get(1, {'url': 'opc.tcp://h:4840', 'use_security': False})
+        c2 = await pool.get('1', {'url': 'opc.tcp://h:4840', 'use_security': False})
+
+    assert c1 is c2
+    assert fake_client.connect.await_count == 1
+
+
+@pytest.mark.asyncio
+async def test_get_with_different_cfg_reuses_client_and_warns():
+    pool = make_pool()
+    fake_client = AsyncMock()
+    with patch('ingestor.connection.pool.Client', return_value=fake_client):
+        await pool.get('srv', {'url': 'opc.tcp://h:4840', 'use_security': False})
+        client = await pool.get('srv', {'url': 'opc.tcp://10.0.0.5:4840', 'use_security': False})
+
+    assert client is fake_client
+    assert fake_client.connect.await_count == 1
+    pool.logger.warning.assert_called_once()
+    assert 'url' in pool.logger.warning.call_args.args[0]
+
+
+@pytest.mark.asyncio
+async def test_secured_connect_failure_invalidates_cert_cache():
+    from unittest.mock import MagicMock
+
+    cert_provider = MagicMock()
+    cert_provider.fetch = AsyncMock(return_value=('cert.pem', 'key.pem'))
+    pool = make_pool(cert_provider=cert_provider)
+    fake_client = AsyncMock()
+    fake_client.connect.side_effect = RuntimeError('BadCertificateUntrusted')
+    with (
+        patch('ingestor.connection.pool.Client', return_value=fake_client),
+        patch('ingestor.connection.pool.apply_security', AsyncMock()),
+    ):
+        with pytest.raises(RuntimeError):
+            await pool.get('srv', {'url': 'opc.tcp://h:4840', 'use_security': True})
+
+    cert_provider.invalidate.assert_called_once()
+
+
+@pytest.mark.asyncio
+async def test_insecure_connect_failure_keeps_cert_cache():
+    from unittest.mock import MagicMock
+
+    cert_provider = MagicMock()
+    pool = make_pool(cert_provider=cert_provider)
+    fake_client = AsyncMock()
+    fake_client.connect.side_effect = RuntimeError('unreachable')
+    with patch('ingestor.connection.pool.Client', return_value=fake_client):
+        with pytest.raises(RuntimeError):
+            await pool.get('srv', {'url': 'opc.tcp://h:4840', 'use_security': False})
+
+    cert_provider.invalidate.assert_not_called()