POST /api/v1/chat (the n8n/Make/Activepieces sync-chat endpoint) verified
session ownership with `_tok_user and _sess_owner and _sess_owner != _tok_user`.
The `_sess_owner and` clause skipped the check entirely whenever the session's
owner was null — so any chat-scoped API token (e.g. a token minted for a paired
mobile device) could pass a legacy/migrated null-owner session id, inject a
message into that session, and read back its conversation history plus reuse
the owner's endpoint credentials.
This is the same `if owner and owner != user` null-owner-bypass pattern that
was already hardened in the gallery, calendar, and notes routes (see
test_null_owner_gates.py) and in session_routes._verify_session_owner. Make
this gate strict and fail closed too: require a resolvable caller and an exact
owner match, mirroring _verify_session_owner. Extract the decision into
_caller_owns_session() and pin it with regression tests.
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