fix(accordion): prevent controlled→uncontrolled flip in single collapsible Accordion

[chatman-media]

Bug

When <Accordion.Root type="single" collapsible> is used in controlled mode with value set to undefined (the natural way to represent "nothing open" in TypeScript), the component malfunctions:

1. Click to open item → works, onValueChange('item-1') fires
2. Click to close → onValueChange('') fires; if the handler stores undefined (setValue(v || undefined)), the next render sees value={undefined}
3. useControllableState treats prop = undefined as uncontrolled, so it falls back to its stale internal state ('item-1') — the item visually re-opens
4. Clicking again tries to close but gets stuck in the same loop

This produces the "3 clicks to close" symptom reported in #3478.

Root cause

useControllableState uses prop !== undefined to decide whether the component is controlled. When a user passes value={undefined} (meaning "no item open"), the component silently becomes uncontrolled and the stale uncontrolled state resurrects the last-open item.

Fix

In AccordionImplSingle, detect whether the value prop was explicitly provided via 'value' in props. When it was, normalize undefined → '' before handing it to useControllableState. This keeps the component in controlled mode for its entire lifetime regardless of what the parent stores.

+ const isValueControlled = 'value' in props;
  const [value, setValue] = useControllableState({
-   prop: valueProp,
+   prop: isValueControlled ? (valueProp ?? '') : valueProp,
    defaultProp: defaultValue ?? '',
    onChange: onValueChange,
    caller: ACCORDION_NAME,
  });

The change is backward-compatible:

• Uncontrolled (<Accordion> with no value prop): 'value' in props is false → behaviour unchanged
• Controlled with a value (value="item-1"): isValueControlled is true, valueProp ?? '' is 'item-1' → behaviour unchanged
• Controlled with nothing open (value={undefined}): normalized to '' → stays controlled, empty value ✓

Test evidence

A new test given a controlled single collapsible Accordion covers the exact failure scenario. It fails without the fix and passes after:

✓ should show the content after the first click
✓ should hide the content after the second click   ← was failing
✓ should show the content again (re-open)          ← was failing

Full test suite: 32 test files, 350 tests — all pass.

Closes #3478

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