fix(feishu): convert markdown headings to div elements in card messages

Markdown heading syntax (#) is not properly rendered in Feishu interactive
cards. Convert headings to div elements with lark_md format (bold text) for
proper display.

- Add _HEADING_RE regex to match markdown headings (h1-h6)
- Add _split_headings() method to parse and convert headings to div elements
- Update _build_card_elements() to process headings before markdown content

nanobot/channels/feishu.py

@@ -166,6 +166,10 @@ class FeishuChannel(BaseChannel):
         re.MULTILINE,
     )
 
+    _HEADING_RE = re.compile(r"^(#{1,6})\s+(.+)$", re.MULTILINE)
+
+    _CODE_BLOCK_RE = re.compile(r"(```[\s\S]*?```)", re.MULTILINE)
+
     @staticmethod
     def _parse_md_table(table_text: str) -> dict | None:
         """Parse a markdown table into a Feishu table element."""
@@ -185,17 +189,52 @@ class FeishuChannel(BaseChannel):
         }
 
     def _build_card_elements(self, content: str) -> list[dict]:
-        """Split content into markdown + table elements for Feishu card."""
+        """Split content into div/markdown + table elements for Feishu card."""
         elements, last_end = [], 0
         for m in self._TABLE_RE.finditer(content):
-            before = content[last_end:m.start()].strip()
-            if before:
-                elements.append({"tag": "markdown", "content": before})
+            before = content[last_end:m.start()]
+            if before.strip():
+                elements.extend(self._split_headings(before))
             elements.append(self._parse_md_table(m.group(1)) or {"tag": "markdown", "content": m.group(1)})
             last_end = m.end()
-        remaining = content[last_end:].strip()
+        remaining = content[last_end:]
+        if remaining.strip():
+            elements.extend(self._split_headings(remaining))
+        return elements or [{"tag": "markdown", "content": content}]
+
+    def _split_headings(self, content: str) -> list[dict]:
+        """Split content by headings, converting headings to div elements."""
+        protected = content
+        code_blocks = []
+        for m in self._CODE_BLOCK_RE.finditer(content):
+            code_blocks.append(m.group(1))
+            protected = protected.replace(m.group(1), f"\x00CODE{len(code_blocks)-1}\x00", 1)
+
+        elements = []
+        last_end = 0
+        for m in self._HEADING_RE.finditer(protected):
+            before = protected[last_end:m.start()].strip()
+            if before:
+                elements.append({"tag": "markdown", "content": before})
+            level = len(m.group(1))
+            text = m.group(2).strip()
+            elements.append({
+                "tag": "div",
+                "text": {
+                    "tag": "lark_md",
+                    "content": f"**{text}**",
+                },
+            })
+            last_end = m.end()
+        remaining = protected[last_end:].strip()
         if remaining:
             elements.append({"tag": "markdown", "content": remaining})
+
+        for i, cb in enumerate(code_blocks):
+            for el in elements:
+                if el.get("tag") == "markdown":
+                    el["content"] = el["content"].replace(f"\x00CODE{i}\x00", cb)
+
         return elements or [{"tag": "markdown", "content": content}]
 
     async def send(self, msg: OutboundMessage) -> None: