`_discord_message_admission()` drops a message that mentions someone other than
the bot when `DISCORD_IGNORE_NO_MENTION` is on (the default) and the channel is
not free-response. It did so without consulting `_in_bot_thread()`, unlike the
other two ingress paths — `_dispatch_recovered_message()` (adapter.py:2292) and
`_handle_message()` (adapter.py:5946). Admission runs on both and returns
`False` unconditionally, so it overrode the thread exemption they grant.
The result was an asymmetry with no obvious cause from the outside: in a thread
the bot had joined, a message with no mention at all was admitted (an empty
`message.mentions` skips the enclosing block), while the same message with one
mention of a third party was dropped. A thread the bot is a participant in is
the one place "addressed to someone else" is least likely to hold.
`thread_require_mention` still gates multi-bot threads, since that check lives
inside `_in_bot_thread()`.
The drop also emitted nothing at any log level, leaving `gateway.log` identical
whether the gate fired or the event never arrived; add a debug line so the two
can be told apart.
Fixes#116568
a0e7fc4a9e