fix(ai): cache OpenRouter tool results

[mteam88]

Summary

• allow Anthropic-style cache markers on trailing OpenAI tool messages
• advance the cached conversation prefix across tool-only turns
• add regression coverage for moving the marker from the earlier user message to the tool result

Related to #6940.

Testing

• npm run check
• HOME="$(mktemp -d)" ./test.sh

[badlogic]

Reviewed. This is safe to merge: OpenRouter accepts cache_control on text blocks in tool messages, and the native Anthropic path already applies the equivalent marker to tool_result blocks. The regression coverage is sufficient. I’m handling the stale docs and ~anthropic/*-latest generator metadata separately.

This comment is AI-generated by /wr