fix(v2/runtime): wrap string error rejections in Error objects

[leaanthony]

Summary

Fixes #4379

Promise rejections from Go binding calls now wrap string errors in proper Error objects instead of rejecting with raw strings.

Problem

When a Go method returns an error, the Wails runtime rejects the JS promise with a plain string. Libraries like TanStack Query expect Error instances (they access error.message), making the bindings incompatible.

Fix

In Callback(), wrap message.error in new Error() when it's a string, and pass through non-string errors as-is.

Testing

• v2/test/4379/calls_test.mjs — 7 tests, all passing:
  • String error → Error instance with message and stack
  • Successful result still works
  • Non-string error objects are passed through as-is

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