fix(telegram): harden detail history html fallback

apps/api/src/services/telegram_gateway.py

@@ -342,7 +342,7 @@ def _telegram_html_chunks(lines: list[str], limit: int = _TELEGRAM_HTML_CHUNK_LI
             current = []
             current_len = 0
         if line_len > limit:
-            chunks.append(line[:limit])
+            chunks.append(_html_safe_plain_chunk(line, limit=limit))
             continue
         current.append(line)
         current_len += line_len
@@ -357,6 +357,17 @@ def _plain_text_from_html(text: str, limit: int = 3900) -> str:
     return html.unescape(plain)[:limit]
 
 
+def _html_safe_plain_chunk(text: str, limit: int) -> str:
+    """Render one overlong HTML line as parse-safe text for HTML mode chunks."""
+    plain = _plain_text_from_html(text, limit=limit)
+    escaped = html.escape(plain)
+    if len(escaped) <= limit:
+        return escaped
+    # Escaping may expand &, <, >. Trim once more after escaping; a partial HTML
+    # entity is still plain text to Telegram, while a partial tag is not.
+    return escaped[:limit]
+
+
 def _sanitize_telegram_error(text: str) -> str:
     """遮蔽 Telegram Bot URL 中的 token，避免例外字串污染 log / trace。"""
     return _TELEGRAM_BOT_URL_RE.sub(r"\1<redacted>", text)
@@ -5920,13 +5931,29 @@ class TelegramGateway:
         Returns:
             dict: API 回應
         """
+        payload_text = text[:500]
+        payload_parse_mode = parse_mode
+        if parse_mode and parse_mode.upper() == "HTML" and len(text) > 500:
+            payload_text = _plain_text_from_html(text, limit=500)
+            payload_parse_mode = None
+
         payload = {
             "chat_id": chat_id or self.alert_chat_id,
-            "text": text[:500],  # SOUL.md 字數限制
-            "parse_mode": parse_mode,
+            "text": payload_text,  # SOUL.md 字數限制
         }
+        if payload_parse_mode:
+            payload["parse_mode"] = payload_parse_mode
 
-        return await self._send_request("sendMessage", payload)
+        try:
+            return await self._send_request("sendMessage", payload)
+        except TelegramGatewayError as exc:
+            if payload_parse_mode and payload_parse_mode.upper() == "HTML" and "HTTP error: 400" in str(exc):
+                fallback_payload = {
+                    "chat_id": chat_id or self.alert_chat_id,
+                    "text": _plain_text_from_html(text, limit=500),
+                }
+                return await self._send_request("sendMessage", fallback_payload)
+            raise
 
     async def _send_html_line_message(
         self,