Feishu's open_id is app-scoped (same user gets different open_ids per
bot app), not a canonical identity. Functionally correct for single-bot
mode but semantically misleading.
- Add comprehensive Feishu identity model documentation to module docstring
- Prefer user_id (tenant-scoped) over open_id (app-scoped) in
_resolve_sender_profile when both are available
- Document bot_open_id usage for @mention matching
- Update user_id_alt comment in SessionSource to be platform-generic
Ref: closes analysis from PR #8388 (closed as over-scoped)