[material_ui] Migrate MaterialApp API doc snippets to {@example} and add unit tests (batch 6)

[dkwingsmt]

This PR addresses batch 6 of the migration detailed in flutter/flutter#188530 (comment).

app.snippet.4.dart‎ contains some non-trivial changes to fix a type error and to make it testable.

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[gemini-code-assist]

Code Review

This pull request refactors the documentation for MaterialApp by replacing inline code snippets with external example files and adds the corresponding example and test files. Feedback was provided to improve test robustness by using more stable widget finders instead of relying on specific text content.

[gemini-code-assist]

Using tester.element(find.text('Home Route')) to obtain a BuildContext for navigation makes the test fragile, as it relies on the specific text content of the UI. A more robust and idiomatic approach in Flutter widget tests is to access the NavigatorState directly using tester.state.

Suggested change

	final BuildContext context = tester.element(find.text('Home Route'));
	Navigator.of(context).pushNamed('/about');
	tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/about');

[dkwingsmt]

I think this isn't significant.

[gemini-code-assist]

Obtaining the BuildContext from a specific text widget makes the test fragile if the text content changes. It is more robust to obtain the context from a structural widget like Scaffold.

Suggested change

	final BuildContext context = tester.element(find.text('MaterialApp Theme'));
	final ThemeData theme = Theme.of(context);
	final BuildContext context = tester.element(find.byType(Scaffold));
	final ThemeData theme = Theme.of(context);

[dkwingsmt]

I think this isn't significant.

[QuncCccccc]

LGTM!

[QuncCccccc]

nit: maybe we can test this against iOS like

Suggested change

	});
	}, variant: TargetPlatformVariant.only(TargetPlatform.iOS));

This test still passes if the snippet's one extra shortcut, const SingleActivator(LogicalKeyboardKey.select): const ActivateIntent(), is removed because the same entry is already in WidgetsApp.defaultShortcuts on Android

[dkwingsmt]

Really good point! I've changed the activating key from Select to keyE, which should be more useful as an example than having the platform set to iOS.