fix: Prevent crash when only one metric name exists (#1806)

Closes HDX-3511

# Summary

This PR fixes a crash that occurs when only one metric name is available. The actual fix is 1 character, and is noted in the comments. The rest of the changes are for unit testing the case and adding an error boundary around the chart editor component.

## Before

https://github.com/user-attachments/assets/e374b36b-0be3-468d-9469-3b1a5c88cd44

## After

https://github.com/user-attachments/assets/50004a07-b13e-4ff5-babb-952e391a8871

## New Error Boundary

If a similar error were to happen again, there is now an ErrorBoundary wrapping the chart editor, so there is no full page crash:

<img width="1757" height="865" alt="Screenshot 2026-02-25 at 3 15 13 PM" src="https://github.com/user-attachments/assets/a2eda617-efcf-4b41-8765-f586d285f50e" />

Author: pulpdrew

.changeset/ten-pans-cough.md

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+---
+"@hyperdx/app": patch
+---
+
+fix: Prevent crash when only one metric name exists