fix(feishu): correct identity model docs and prefer tenant-scoped user_id

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)

tests/gateway/test_feishu.py

@@ -1537,7 +1537,7 @@ class TestAdapterBehavior(unittest.TestCase):
         adapter._dispatch_inbound_event.assert_awaited_once()
         event = adapter._dispatch_inbound_event.await_args.args[0]
         self.assertEqual(event.message_type, MessageType.TEXT)
-        self.assertEqual(event.source.user_id, "ou_user")
+        self.assertEqual(event.source.user_id, "u_user")  # tenant-scoped user_id preferred over app-scoped open_id
         self.assertEqual(event.source.user_name, "张三")
         self.assertEqual(event.source.user_id_alt, "on_union")
         self.assertEqual(event.source.chat_name, "Feishu DM")