* fix(feishu): remove incorrect oc_ prefix assumption in resolveFeishuSession
- Feishu oc_ is a generic chat_id that can represent both groups and DMs
- Must use chat_mode field from API to distinguish, not ID prefix
- Only ou_/on_ prefixes reliably indicate user IDs (always DM)
- Fixes session misrouting for DMs with oc_ chat IDs
This bug caused DM messages with oc_ chat_ids to be incorrectly
created as group sessions, breaking session isolation and routing.
* docs: update Feishu ID format comment to reflect oc_ ambiguity
The previous comment incorrectly stated oc_ is always a group chat.
This update clarifies that oc_ chat_ids can be either groups or DMs,
and explicit prefixes (dm:/group:) should be used to distinguish.
* feishu: add regression coverage for oc session routing
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Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>
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