fix(delivery): drop env-var knob, flag all chunking adapters

Follow-up to ScotterMonk's cron-truncation fix:

- Remove HERMES_DELIVERY_MAX_PLATFORM_OUTPUT env var. Behavioral config
  belongs in config.yaml, not a new HERMES_* env var (.env is secrets
  only). The actual bug is fixed entirely by the adapter-aware skip; the
  configurable cap was unneeded scope. MAX_PLATFORM_OUTPUT is a constant
  again, collapsing the max_output=0 disable branch and the
  audit-vs-truncation threshold divergence.
- Flag the remaining verified-chunking adapters (slack, matrix, feishu,
  mattermost, teams, whatsapp, whatsapp_cloud, weixin, bluebubbles,
  yuanbao) with splits_long_messages=True so the fix covers the whole
  bug class, not just Discord/Telegram. Each verified to chunk in its
  own send() via truncate_message().
- SMS deliberately left False: it chunks for normal replies but a
  multi-segment cron blast is cost-bearing; the 4000-cap + file save is
  the safer default there.
- Update tests: drop the two env-override tests, add a test asserting a
  save failure during truncation (non-chunking) propagates.

plugins/platforms/feishu/adapter.py

@@ -1410,6 +1410,7 @@ class FeishuAdapter(BasePlatformAdapter):
     """Feishu/Lark bot adapter."""
 
     supports_code_blocks = True  # Feishu renders fenced code blocks
+    splits_long_messages = True  # send() chunks via truncate_message(MAX_MESSAGE_LENGTH)
 
     MAX_MESSAGE_LENGTH = 8000
     # Max distinct chat IDs retained in _chat_locks before LRU eviction kicks in.