fix(accordion): let content overflow the box once open (ARTESCA-17819)

[JeanMarcMilletScality]

TL;DR — Content opened inside an Accordion (a Select menu, dropdown or tooltip) is no longer clipped by the box: once the accordion has finished opening, its content is allowed to overflow.

Context

The Accordion animates with overflow: hidden so the height transition clips cleanly. But that also clips anything a child legitimately renders outside the box — most visibly a Select/dropdown menu opened near the bottom, which got cut off at the accordion border. ARTESCA-17819.

Approach

Keep overflow: hidden only while the open/collapse height transition runs (so the animation still clips), then flip to overflow: visible once opening is done:

• an isExpanded state gates overflow and is set on the content's own height transitionEnd (guarded with e.target === e.currentTarget so a descendant's bubbled height transition doesn't flip it early);
• isExpanded initializes to open, so an initially-open accordion is visible from mount and never depends on a transition that wouldn't fire on first paint;
• closing resets isExpanded, so the collapse animation clips again.

⚠️ Merge note — reconcile with #1170

This PR and #1170 (fix(accordion): follow content height) both touch the Accordion's overflow/height handling but solve different bugs:

	this PR	#1170
menu/popover escapes the box when open	✅	❌
box resizes when content grows/shrinks after open	❌ (locked height → content spills)	✅

They're complementary — combined, the box follows its content height (nothing spills) and overflow is visible (popovers escape). Whoever merges second must integrate both mechanisms rather than let one overwrite the other.

How to test

• Storybook → Accordion → WithSelectAtBottom: open the Select near the bottom; its menu overflows past the accordion border and stays fully visible/scrollable.
• Jest — npx jest src/lib/components/accordion

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