fix(weixin): split chatty short replies into separate bubbles, keep structured content together

Add content-aware splitting to compact mode: short chat-like exchanges
(2-6 short lines without headings/lists/quotes) get separate message
bubbles for a natural chat feel, while structured content (tables,
headings with body, numbered lists) stays in a single message.

Cherry-picked from PR #7587 by bravohenry, adapted to the compact/legacy
split_per_line architecture from #7903.

gateway/platforms/weixin.py

@@ -734,6 +734,42 @@ def _split_delivery_units_for_weixin(content: str) -> List[str]:
     return [unit for unit in units if unit]
 
 
+def _looks_like_chatty_line_for_weixin(line: str) -> bool:
+    """Return True when a line looks like a standalone chat utterance."""
+    stripped = line.strip()
+    if not stripped:
+        return False
+    if len(stripped) > 48:
+        return False
+    if line.startswith((" ", "\t")):
+        return False
+    if stripped.startswith((">", "-", "*", "【")):
+        return False
+    if re.match(r"^\*\*[^*]+\*\*$", stripped):
+        return False
+    if re.match(r"^\d+\.\s", stripped):
+        return False
+    return True
+
+
+def _looks_like_heading_line_for_weixin(line: str) -> bool:
+    """Return True when a short line behaves like a plain-text heading."""
+    stripped = line.strip()
+    if not stripped:
+        return False
+    return len(stripped) <= 24 and stripped.endswith((":", "："))
+
+
+def _should_split_short_chat_block_for_weixin(block: str) -> bool:
+    """Split only chat-like multiline blocks into separate bubbles."""
+    lines = [line for line in block.splitlines() if line.strip()]
+    if not 2 <= len(lines) <= 6:
+        return False
+    if _looks_like_heading_line_for_weixin(lines[0]):
+        return False
+    return all(_looks_like_chatty_line_for_weixin(line) for line in lines)
+
+
 def _pack_markdown_blocks_for_weixin(content: str, max_length: int) -> List[str]:
     if len(content) <= max_length:
         return [content]
@@ -787,9 +823,15 @@ def _split_text_for_weixin_delivery(
             chunks.extend(_pack_markdown_blocks_for_weixin(unit, max_length))
         return chunks or [content]
 
-    # Compact (default): single message when under the limit.
+    # Compact (default): single message when under the limit — unless the
+    # content looks like a short chatty exchange, in which case split into
+    # separate bubbles for a more natural chat feel.
     if len(content) <= max_length:
-        return [content]
+        return (
+            _split_delivery_units_for_weixin(content)
+            if _should_split_short_chat_block_for_weixin(content)
+            else [content]
+        )
     return _pack_markdown_blocks_for_weixin(content, max_length) or [content]