Handle Anthropic invalid_request_error in Duo Chat

Problem

Some requests to V2 Duo Chat encountered the exception invalid_request_error in Anthropic.

Error message: all messages must have non-empty content except for the optional final assistant message

See the comments below for more details.

This also could result in A1002.

Proposal

• Investigate why it happened.
• Fix the root cause.

Root cause

Chat history (persisted messages in Redis) are likely corrupted.

Workaround

Clean up the chat history by /reset or /clear.