fix(mock-doc): add missing part setter to MockElement

[hyyan]

What is the current behavior?

MockElement.part has a getter but no setter. When setAccessor does elm.part = "value" on custom elements, it overwrites the getter with a plain string on the instance. setAttribute is never called and the part attribute is silently lost in spec test snapshots.

GitHub Issue Number: #6611

What is the new behavior?

Closes #6611

Added a set part() setter to MockElement, matching how className/classList work. The part attribute now correctly appears on custom elements in spec tests.

Documentation

N/A

Does this introduce a breaking change?

• Yes
• No

Testing

Added some regression tests

closes #6611

[Copilot]

Pull request overview

This PR fixes a bug where the part attribute was silently lost on custom elements in spec tests. The issue occurred because MockElement.part had only a getter without a corresponding setter, causing setAccessor to overwrite the getter with a plain string value instead of properly setting the attribute.

Changes:

• Added a set part() setter to MockElement that calls setAttributeNS to properly handle the part attribute
• Added regression tests verifying that the part attribute is correctly set, updated, and removed on custom elements

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/mock-doc/node.ts	Added setter for part property to properly set the attribute via setAttributeNS
src/runtime/vdom/test/set-accessor.spec.ts	Added three test cases covering setting, updating, and removing the part attribute

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[johnjenkins]

LGTM!