Closes #50569 Before you mark this PR as ready for review, make sure that you have: - [x] Added a solid test coverage and/or screenshots from doing manual testing - [x] Done a self-review taking into account security and performance aspects - [x] Aligned any UI changes with the [UI checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist) Previously, OpenRouter responses containing only usage data (without any choices) would cause an error. Now the mapper properly emits usage updates for these events without failing. Release Notes: - Fixed an error when OpenRouter returns a usage-only event with empty choices.