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>

ingestor/io/slots.py

@@ -11,6 +11,8 @@ any order (SIENTIAPDE-1974):
 """
 
 SERVER_FIELDS = ('server_id', 'url', 'server_uri', 'use_security', 'subscription_period_ms')
+# What a connection/subscription needs (the pool keys clients by server_id itself).
+CONNECTION_FIELDS = tuple(f for f in SERVER_FIELDS if f != 'server_id')
 
 
 def _parse_names(address: str, tag: dict) -> list[dict]: