[material_ui] Migrate Checkbox, CheckboxListTile, Radio, and RadioListTile API doc snippets to {@example} (batch 9)

[dkwingsmt]

This PR addresses batch 9 of the migration detailed in flutter/flutter#188530 (comment).

Non-trivial changes:

• None

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[gemini-code-assist]

Code Review

This pull request extracts inline code snippets from the documentation of Checkbox, CheckboxListTile, Radio, and RadioListTile into dedicated snippet files within the example package, and introduces corresponding widget tests. Feedback on the changes suggests simplifying the Checkbox snippet by removing private getters inside the snippet body to improve the readability and usability of the generated API documentation.

[gemini-code-assist]

The use of private getters _value and _nullableOnChanged in the snippet body makes the generated API documentation less readable and harder to copy-paste directly, as these getters are defined outside the #region body and are not standard. Since onChanged is already nullable and value is always true in the tests, we can use true and onChanged directly in the Checkbox widget.

Suggested change

	final ValueChanged<bool?>? onChanged;
	ValueChanged<bool?>? get _nullableOnChanged => onChanged;
	bool get _value => true;
	@override
	Widget build(BuildContext context) {
	return
	// #region body
	Checkbox(
	value: _value,
	// If `onChanged` is null, this checkbox is disabled.
	// If `onChanged` is not null, this checkbox is enabled.
	onChanged: _nullableOnChanged,
	final ValueChanged<bool?>? onChanged;
	@override
	Widget build(BuildContext context) {
	return
	// #region body
	Checkbox(
	value: true,
	// If `onChanged` is null, this checkbox is disabled.
	// If `onChanged` is not null, this checkbox is enabled.
	onChanged: onChanged,

[QuncCccccc]

LGTM!