Fix session export 500 on multimodal/None message content (#1984)

txt/html/md export joined and string-munged message.content directly, so a
multimodal turn (content is a list of blocks) crashed export with a TypeError
on join (txt) / AttributeError on .replace (html), and None content (tool-only
assistant turns) rendered as the literal 'None'. Add a _content_to_text helper
that flattens string/list/None to plain text and apply it at the three export
sites. JSON export is unchanged (it serializes structured content correctly).
Plain-string content is returned unchanged, so existing exports are identical.

Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>

routes/session_routes.py

@@ -37,6 +37,26 @@ def _public_model(name: str, model: str) -> str:
     return model
 
 
+def _content_to_text(content) -> str:
+    """Flatten a message's content to plain text for text-based exports.
+
+    History entries carry three shapes: a plain string, a multimodal list of
+    content blocks (vision/image attachments), or None (assistant turns that
+    persisted only native tool_calls). The txt/html/md exporters join and
+    string-munge this value, so a list crashed the export (TypeError on join,
+    AttributeError on .replace) and None rendered as the literal "None".
+    Coerce to the text blocks, returning "" for anything without text.
+    """
+    if isinstance(content, str):
+        return content
+    if isinstance(content, list):
+        return "\n".join(
+            b.get("text", "") for b in content
+            if isinstance(b, dict) and b.get("text")
+        )
+    return ""
+
+
 def _verify_session_owner(request: Request, session_id: str, session_manager=None):
     """Verify the current user owns the session. Raises 404 if not.
 
@@ -708,7 +728,7 @@ def setup_session_routes(session_manager: SessionManager, config: dict, webhook_
             lines = []
             for m in session.history:
                 lines.append(f"[{m.role.upper()}]")
-                lines.append(m.content)
+                lines.append(_content_to_text(m.content))
                 lines.append("")
             out_name = filename or f"conversation_{safe_name}_{timestamp}.txt"
             return Response(
@@ -731,7 +751,7 @@ def setup_session_routes(session_manager: SessionManager, config: dict, webhook_
             ]
             for m in session.history:
                 cls = "user" if m.role == "user" else "ai"
-                content = m.content.replace("&", "&amp;").replace("<", "&lt;").replace(">", "&gt;")
+                content = _content_to_text(m.content).replace("&", "&amp;").replace("<", "&lt;").replace(">", "&gt;")
                 content = content.replace("\n", "<br>")
                 html_parts.append(f'<div class="msg {cls}"><div class="role">{m.role}</div>{content}</div>')
             html_parts.append("</body></html>")
@@ -750,7 +770,7 @@ def setup_session_routes(session_manager: SessionManager, config: dict, webhook_
         markdown_lines.append("\n---\n")
         for message in session.history:
             role = message.role.upper()
-            content = message.content
+            content = _content_to_text(message.content)
             markdown_lines.append(f"### {role}")
             markdown_lines.append(f"{content}\n")
             markdown_lines.append("---\n")