Port from openclaw/openclaw#64796: Per MSC3952 / Matrix v1.7, the
m.mentions.user_ids field is the authoritative mention signal. Non-
OpenClaw Matrix clients (Element, matrix-bot-sdk bots, etc.) commonly
send messages with proper m.mentions.user_ids metadata but without
duplicating the @bot text in the message body.
Before this change, _is_bot_mentioned() relied entirely on text-based
detection (body string matching and HTML pill detection), causing
messages from these clients to be silently dropped when
MATRIX_REQUIRE_MENTION=true.
Now, if the bot's user_id appears in m.mentions.user_ids, that alone
is sufficient to register a mention — matching the Matrix spec.
Text-based fallback remains for backwards compatibility with older
clients that don't populate m.mentions.
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