refactor(telegram): use entity-only mention detection

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).

gateway/platforms/telegram.py

@@ -2258,23 +2258,27 @@ class TelegramAdapter(BasePlatformAdapter):
 
         bot_username = (getattr(self._bot, "username", None) or "").lstrip("@").lower()
         bot_id = getattr(self._bot, "id", None)
+        expected = f"@{bot_username}" if bot_username else None
 
         def _iter_sources():
             yield getattr(message, "text", None) or "", getattr(message, "entities", None) or []
             yield getattr(message, "caption", None) or "", getattr(message, "caption_entities", None) or []
 
+        # Telegram parses mentions server-side and emits MessageEntity objects
+        # (type=mention for @username, type=text_mention for @FirstName targeting
+        # a user without a public username). Only those entities are authoritative —
+        # raw substring matches like "foo@hermes_bot.example" are not mentions
+        # (bug #12545). Entities also correctly handle @handles inside URLs, code
+        # blocks, and quoted text, where a regex scan would over-match.
         for source_text, entities in _iter_sources():
-            if bot_username:
-                if re.search(rf'(?<!\w)@{re.escape(bot_username)}(?!\w)', source_text, re.IGNORECASE):
-                    return True
             for entity in entities:
                 entity_type = str(getattr(entity, "type", "")).split(".")[-1].lower()
-                if entity_type == "mention" and bot_username:
+                if entity_type == "mention" and expected:
                     offset = int(getattr(entity, "offset", -1))
                     length = int(getattr(entity, "length", 0))
                     if offset < 0 or length <= 0:
                         continue
-                    if source_text[offset:offset + length].strip().lower() == f"@{bot_username}":
+                    if source_text[offset:offset + length].strip().lower() == expected:
                         return True
                 elif entity_type == "text_mention":
                     user = getattr(entity, "user", None)