fix: AccordionTrigger className should append, not replace

[Devil1716]

Summary

• AccordionTrigger's className was applied before {...trigger.props}, so the custom class overwrote Accordion__trigger
• Move the merged className after the spread so both Accordion__trigger and the consumer class are kept
• Strengthen the unit test to assert Accordion__trigger, ExpandCollapse__trigger, and the custom class are all present

Closes #2480

Test plan

• node scripts/buildIconTypes.js && pnpm exec jest src/components/Accordion/index.test.tsx --no-coverage (21 passed)

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