fix: handle message length overflow in streaming mode (#1783)

Stream consumer now splits messages that exceed the platform's
MAX_MESSAGE_LENGTH. When accumulated text grows past the safe limit,
the current message is finalized and a new message is started for the
overflow — same as how normal sends chunk long responses.

Split point prefers line boundaries (rfind newline) for clean breaks.
Works for all platforms (Telegram 4096, Discord 2000, etc.) by reading
the adapter's MAX_MESSAGE_LENGTH at runtime.

Also added a safety net in the Telegram adapter: if edit_message_text
still hits MESSAGE_TOO_LONG (e.g. markdown formatting expansion), it
truncates and returns success so the stream consumer doesn't die.

Co-authored-by: Test <test@test.com>

gateway/platforms/telegram.py

@@ -430,6 +430,20 @@ class TelegramAdapter(BasePlatformAdapter):
             # "Message is not modified" — content identical, treat as success
             if "not modified" in err_str:
                 return SendResult(success=True, message_id=message_id)
+            # Message too long — content exceeded 4096 chars (e.g. during
+            # streaming).  Truncate and succeed so the stream consumer can
+            # split the overflow into a new message instead of dying.
+            if "message_too_long" in err_str or "too long" in err_str:
+                truncated = content[: self.MAX_MESSAGE_LENGTH - 20] + "…"
+                try:
+                    await self._bot.edit_message_text(
+                        chat_id=int(chat_id),
+                        message_id=int(message_id),
+                        text=truncated,
+                    )
+                except Exception:
+                    pass  # best-effort truncation
+                return SendResult(success=True, message_id=message_id)
             # Flood control / RetryAfter — back off and retry once
             retry_after = getattr(e, "retry_after", None)
             if retry_after is not None or "retry after" in err_str: