fix: strip webui metadata from messages before sending to LLM API (#67)

The webui stores display-only fields on messages (attachments, timestamp,
_ts) for UI rendering. These leaked into the conversation_history passed
to AIAgent.run_conversation(). Most providers ignore unknown fields, but
Z.AI/GLM tries to deserialize 'attachments' as its native ChatAttachments
type, causing HTTP 400 on every subsequent message after an image upload.

Fix: _sanitize_messages_for_api() creates a clean copy with only
API-standard keys (role, content, tool_calls, tool_call_id, name,
refusal) before passing to run_conversation(). Applied to both the
streaming path (streaming.py) and non-streaming path (routes.py).

Closes #66

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/routes.py

@@ -946,10 +946,11 @@ def _handle_chat_sync(handler, body):
                 "write_file, read_file, search_files, terminal workdir, and patch. "
                 "Never fall back to a hardcoded path when this tag is present."
             )
+            from api.streaming import _sanitize_messages_for_api
             result = agent.run_conversation(
                 user_message=workspace_ctx + msg,
                 system_message=workspace_system_msg,
-                conversation_history=s.messages,
+                conversation_history=_sanitize_messages_for_api(s.messages),
                 task_id=s.session_id,
                 persist_user_message=msg,
             )