bug: `part` attribute not set on custom elements in spec tests

[hyyan]

Prerequisites

• I have read the Contributing Guidelines.
• I agree to follow the Code of Conduct.
• I have searched for existing issues that already report this problem, without success.

Stencil Version

v4.38.2

Current Behavior

When a component renders a child custom element with a part attribute, the attribute is silently lost in spec test output. Snapshots show the part missing entirely.

This happens because MockElement.part (added in #6423) has a getter but no setter. setAccessor sees 'part' in elm is true and does elm.part = "value", which overwrites the getter with a plain string on the instance. setAttribute is never called.

Expected Behavior

The part attribute should appear on custom elements in spec test snapshots, matching real browser behavior.

System Info

Steps to Reproduce

1. Create a component that renders a child custom element with part
2. Write a spec test with newSpecPage and snapshot the output
3. The part attribute is missing from the child custom element

Code Reproduction URL

N/A

Additional Information

N/A