fix(devtools-kit): guard state editor against undefined path traversal

[arena-ai-coding-agent]

Summary

Editing state from the component inspector (e.g. a Pinia store) throws an uncaught:

TypeError: this.refEditor.get(...) is undefined

Cause

In StateEditor.set(), the path used to locate the value may not exist on the target object. After traversing such a path, object becomes undefined, and the code then evaluates:

const item = this.refEditor.get(object)[field]

which reads a property off undefined and throws a TypeError. This is reproducible when the inspector passes a path that the component proxy does not have (e.g. a Pinia state path from the Components tab).

Fix

Stop the edit gracefully when the path does not resolve, instead of throwing. An early if (object == null) return is added both inside the traversal loop and before the final field access. Legitimate edits are unaffected.

Tests

Added a regression test asserting that editing via a non-existing nested path does not throw. The full suite passes (86 tests) along with type-check, lint, and build.

Closes vuejs#906