fix: use UTF-16 length for Telegram stream consumer message splitting

The stream consumer measured message length using Python's len() (Unicode
code points), but Telegram's actual limit is in UTF-16 code units. This
caused messages with supplementary characters (emoji, CJK, etc.) to exceed
Telegram's 4096-character limit, resulting in truncated messages with
formatting artifacts.

Changes:
- Add message_len_fn property to BasePlatformAdapter (defaults to len)
- Override in TelegramAdapter to return utf16_len
- Stream consumer uses adapter.message_len_fn for:
  - safe_limit calculation
  - overflow detection
  - truncate_message calls
  - split point calculation (via _custom_unit_to_cp)
  - fallback final send chunking

Fixes truncated messages with black square artifacts on Telegram when
the model generates responses containing multi-byte Unicode characters.

gateway/platforms/base.py

@@ -1311,6 +1311,15 @@ class BasePlatformAdapter(ABC):
         # _keep_typing skips send_typing when the chat_id is in this set.
         self._typing_paused: set = set()
 
+    @property
+    def message_len_fn(self) -> Callable[[str], int]:
+        """Return the length function for measuring message size on this platform.
+
+        Override in adapters whose platform counts characters differently from
+        Python ``len`` (e.g. Telegram counts UTF-16 code units).
+        """
+        return len
+
     @property
     def has_fatal_error(self) -> bool:
         return self._fatal_error_message is not None