fix(form-elements): add element internals mock stencil unit test

[QuintonJason]

Description

Summary

• Add a lightweight ElementInternals mock to Stencil unit tests for form-associated components (input/textarea/select/checkbox/switch/combobox).
• Prevents CI failures where Jest treats the setFormValue access warning as a test error.

Motivation and Context

CI was failing on unit tests due to ElementInternals.setFormValue being unimplemented in Stencil’s mock DOM. Mocking attachInternals in unit tests avoids console errors while keeping production code unchanged.

Dependencies

• None

Fixes #(issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• unit tests
• e2e tests
• accessibility tests
• tested manually
• other:

Test Configuration:

• Pine versions:
• OS: darwin 25.1.0
• Browsers:
• Screen readers:
• Misc: Suggested local repro: cd libs/core && CI=1 npm test -- --ci

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• Design has QA'ed and approved this PR

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