[material_ui] Move over more API samples

[dkwingsmt]

This adds missing samples referenced in the material_ui API docs.
Some of these were misplaced in the widgets/ API directory in flutter/flutter, I will push up a PR to reflect the move there as well so we do not lose track of it.

Towards flutter/flutter#172936 and flutter/flutter#172937

Related:

• [material_ui] Migrate api sample code to @example dartdoc directive #12078, which migrates samples in material_ui and depends on this PR.
• [cupertino_ui] Move over more API samples #12086, which moves similar missing sample files for cupertino_ui

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[gemini-code-assist]

Code Review

This pull request adds new example applications and tests for TextField custom magnifiers and tap regions, updating the inline documentation in text_field.dart to reference these examples. Feedback on the changes highlights a critical issue where a TextEditingController is instantiated inside a StatelessWidget's build method, which can cause state resets and memory leaks. Additionally, it is recommended to declare the controller as final in the tap region example, remove unused generic type parameters in the magnifier tests, and avoid relying on the private _Editable class name in the tap region tests.

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[Piinks]

Ah the period I included is not meant to be there in the license header 😓

[Piinks]

Sorry 😞

[Piinks]

LGTM