fix(pds-accordion): add accordion-icon part for styling

[pixelflips]

Description

Added a CSS part attribute to the icon element in the pds-accordion component to allow external styling via the ::part() pseudo-element.

This change exposes the accordion's chevron icon as accordion-icon, enabling developers to customize the icon's appearance from outside the shadow DOM using standard CSS parts syntax.

The component now exposes three parts for styling:

• accordion-button - The summary/trigger button
• accordion-body - The expandable content area
• accordion-icon - The chevron icon (newly added)

Fixes: https://kajabi.atlassian.net/browse/DSS-1566

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• unit tests - Updated all existing unit test snapshots to include the new part="accordion-icon" attribute
• tested manually

Test Configuration:

• Pine versions:
• OS:
• Browsers:
• Screen readers:
• Misc:

Checklist:

If not applicable, leave options unchecked.

• 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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[teenwolfblitzer]

👍 Looks good, thanks for this!