chore(biome): downgrade noNonNullAssertion to warning

Non-null assertions are sometimes necessary after length checks
where TypeScript can't infer the value exists. Treat as warnings
rather than errors to avoid noise while maintaining visibility.

Author: jgpruitt

biome.json

@@ -16,7 +16,10 @@
   "linter": {
     "enabled": true,
     "rules": {
-      "recommended": true
+      "recommended": true,
+      "style": {
+        "noNonNullAssertion": "warn"
+      }
     }
   },
   "overrides": [