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.
c0da5d09a6