[Tek-Lizzy]

Summary

Guard against successful token-balance simulations that omit their result. Instead of causing a TypeError, the balance read now throws a clear error: “Balance read did not return a value.” Added a regression test for this response.

Linked issue

Closes #500

What was tested

• git diff --check passed.
• Editor diagnostics reported no errors in the changed files.
• The focused test and typecheck could not be completed because dependencies were unavailable after the interrupted npm ci. Lint was not run.

Checklist

• npm test passes
• npm run typecheck passes
• npm run lint passes
• Browser testing not applicable; this change does not affect the UI.