Replaces the word-boundary regex scan with pure MessageEntity-based
detection. Telegram's server emits MENTION entities for real @username
mentions and TEXT_MENTION entities for @FirstName mentions; the text-
scanning fallback was both redundant (entities are always present for
real mentions) and broken (matched raw substrings like email addresses,
URLs, code-block contents, and forwarded literal text).
Entity-only detection:
- Closes bug #12545 ("foo@hermes_bot.example" false positive).
- Also fixes edge cases the regex fix would still miss: @handles inside
URLs and code blocks, where Telegram does not emit mention entities.
Tests rewritten to exercise realistic Telegram payloads (real mentions
carry entities; substring false positives don't).
