fix: #3607044 Append a trailing user message when a conversation ends with a non-user role

Rajab Natshahrequested to merge
issue/ai_provider_anthropic-3607044:3607044-chat-requests-rejected into 1.2.x

Fixes #3607044.

AnthropicProvider::chat() forwards messages as-is; Anthropic rejects a conversation whose last message is not from the user ("the conversation must end with a user message" / no assistant prefill). The AI Agents multi-agent handoff (e.g. Drupal Canvas AI) can end a conversation with a trailing assistant or system message, producing a generic "Something went wrong" in Canvas AI.

This overrides chat() to append a minimal trailing user message ("Continue.") whenever the last message role is not user (covering both trailing assistant and system).

Tested on Drupal 11.3, ai_provider_anthropic 1.2.x with ai_agents + Drupal Canvas AI: the prefill error no longer occurs, normal chat unaffected.

AI usage disclosure (per https://www.drupal.org/docs/develop/issues/issue-procedures-and-etiquette/policy-on-the-use-of-ai-when-contributing-to-drupal): AI-Generated: Yes — the patch and this writeup were drafted with AI (Claude) assistance. I, Rajab Natshah, reviewed, tested, and take full responsibility for the correctness, security, and GPL compatibility of this change.

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Edited by Rajab Natshah

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