Chat: requests rejected when the conversation ends with a non-user (assistant/system) message

Problem/Motivation

When a conversation sent to Anthropic ends with a non-user message (a trailing assistant or system role), the request is rejected:

the conversation must end with a user message (does not support assistant message prefill).

This is hit via the AI Agents multi-agent handoff (e.g. Drupal Canvas AI), which can build a conversation ending in roles "… assistant, system". The user sees a generic "Something went wrong" in Canvas AI and the build fails.

Cause: AnthropicProvider::chat() forwards the messages as-is; Anthropic requires the final message to have the user role.

Steps to reproduce

1. Configure Anthropic as the provider for AI Agents / Drupal Canvas AI.
2. Trigger a flow where the handoff ends the conversation with an assistant or system message (e.g. a Canvas AI build).
3. The request fails with the "must end with a user message" / assistant-prefill error.

Proposed resolution

Override chat() to append a minimal trailing user message (e.g. "Continue.") whenever the last message role is not user (covering both a trailing assistant and system message). MR to follow.

AI usage disclosure (per the Drupal AI contribution policy): AI-Generated: Yes — the patch and this writeup were drafted with AI (Claude) assistance. I, Rajab Natshah, reviewed, tested on Drupal 11.3 / ai_provider_anthropic 1.2.x, and take full responsibility for the correctness, security, and GPL compatibility of this change.

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