Models occasionally emit NO_REPLY with trailing underscores, quotes,
markdown emphasis, or short explanations (e.g. `NO_REPLY_`,
`**NO_REPLY**`, `NO_REPLY: offline`). Previous word-boundary check
treated `_` as a word char so `NO_REPLY_` slipped through and was
delivered to end users.
Rewrite IsSilentReply to strip decorative wrappers, then prefix-match
case-insensitively. Token is only rejected when glued to another
word character (e.g. `NO_REPLYING`). Intentional divergence from
upstream #19537 — documented in the doc comment.
