* fix(cache): inject cache_control into system prompt for OpenRouter Anthropic
Add onPayload wrapper that injects cache_control: { type: "ephemeral" }
into the system/developer message content for OpenRouter requests routed
to Anthropic models. The system prompt is typically ~18k tokens and was
being re-processed on every request without caching.
Fixes#15151
* Changelog: add OpenRouter note for #17473
---------
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>
