composable AddSelect: update Storybook patterns and docs to reflect composable API changes

[szinta]

All the changes listed in #9718 require corresponding updates to the AddSelect Storybook stories, MDX documentation, patterns and tests so that patterns shown to consumers stay accurate and runnable.

Scope

Update packages/ibm-products/src/components/AddSelect/next/ :

next/AddSelect.stories.tsx

Change from parent issue	Story update required
Issue 1 — onNavigate removed from root context	Remove any story that passes onNavigate to root AddSelect; ensure navigation examples wire it through AddSelect.Column only
Issue 2 — multi removed from root context	Remove any multi prop set on root AddSelect; move it to the relevant AddSelect.Column instances
Issue 3 — search state renamed + JSDoc added	Update story arg descriptions for onSearch on both Body and Column to reflect their scoped documentation
Issue 5 — navIndicatorLabel new prop on AddSelectRow	Add navIndicatorLabel as an argType in the AddSelectRow story with a descriptive default
Issue 8 — itemDetails type changed to Array<{ label, value }>	Update all story data (e.g. sampleItems, summaryItems) that use itemDetails from Record<string, unknown> to Array<{ label: string; value: string | number }>
Issue 10 — SelectionSummary selectedItems + renderItem props removed; selectedItemCount added	Rewrite the AddSelectSelectionSummary story to use composable AddSelect.SelectionSummaryItem children; replace selectedItems with selectedItemCount prop
Issue 12 — open prop added to AddSelectItemPanel	Add open as a boolean argType in the AddSelectItemPanel story; demonstrate show/hide toggle

next/AddSelect.mdx

• Update all code examples in the MDX docs to reflect the same API changes listed above.
• Add a note under AddSelectBody and AddSelectColumn clarifying that global search and column search are independent (from issue 3).
• Add a note under AddSelectSelectionSummary documenting that the component is children-only (from issue 10).

next/AddSelect.test.js

• Update the ControlledAddSelect test helper if it passes multi or onNavigate to root AddSelect.
• Update any test fixtures that use itemDetails as a Record to the new Array<{ label, value }> shape.
• Update SelectionSummary tests that rely on selectedItems or renderItem props.

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Acceptance criteria

• All stories in AddSelect.stories.tsx run without errors after parent issue changes are applied
• AddSelect.mdx code examples reflect the updated API (no references to removed props)
• AddSelect.test.js test fixtures updated; all tests pass
• No story or doc still references selectedItems or renderItem on SelectionSummary
• No story or doc still passes multi or onNavigate to the root AddSelect
• New navIndicatorLabel and open props are demonstrated in their respective stories
• itemDetails sample data uses Array<{ label, value }> shape throughout