fix: add default for adapter_name in ThreadImpl.from_json

Consistency with ChannelImpl: use empty string default when neither
camelCase nor snake_case key is present. Addresses Codex review comment.

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

Author: patrick-chinchill

src/chat_sdk/thread.py

@@ -724,7 +724,7 @@ def from_json(
         thread = cls(
             _ThreadImplConfig(
                 id=data["id"],
-                adapter_name=data.get("adapterName") or data.get("adapter_name"),
+                adapter_name=data.get("adapterName") or data.get("adapter_name", ""),
                 channel_id=data.get("channelId") or data.get("channel_id", ""),
                 channel_visibility=data.get("channelVisibility") or data.get("channel_visibility", "unknown"),
                 current_message=current_msg,