Make the Material fallback theme follow defaultTargetPlatform - #194256

**Port of: flutter/flutter#194256

**Issue:** `Theme.of(context)` falls back to a `ThemeData` built once per isolate (`Theme._kFallbackTheme`) when there is no `Theme` ancestor. `ThemeData()` captures `defaultTargetPlatform` at construction, so the fallback's platform is frozen to whatever platform was in effect the first time a theme-less widget called `Theme.of(context)`. Under a `TargetPlatformVariant`, `debugDefaultTargetPlatformOverride` every later theme-less subtree keeps that first platform's selection controls, toolbar, cursor and other adaptive behavior. In a shuffled test run this makes theme-less tests depend on test order: with seed 20261008, text_field_test.dart fails 45 tests that pass in declaration order.

**Fix:** Rebuild the cached fallback whenever its platform no longer matches `defaultTargetPlatform`, so a theme-less subtree adapts to the current platform the same way a `MaterialApp` does. `Theme`s built with an explicit platform are unaffected; only the fallback is compared. Adds a regression test in `theme_test.dart` that fails on master for every non-android variant.

With the fallback following the variant, the two handle-drag tests in `text_field_test.dart` fail because they had only ever run against Android handles: desktop platforms draw no selection handles, and the Cupertino start handle sits above the line. These tests have been adjusted; restricted to the platforms that draw handles, the iOS start handle is grabbed where it actually is, the Fuchsia toolbar that covers the handles is hidden, and a test asserts that desktop platforms draw no handles. The orphaned no-shuffle tag is removed.

**Other alternatives considered:**
- `text_field_test.dart` only fix, tests explicitly install their own theme so `Theme.of(context)` does not return leaky fallback. Decided against this since it wouldn't fix this leak for all users as reported in #83788

Fixes [#83788](flutter/flutter#83788)
Fixes [#194008](flutter/flutter#194008)

## Pre-Review Checklist

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…utter#194256)

**Issue:** `Theme.of(context)` falls back to a `ThemeData` built once
per isolate (`Theme._kFallbackTheme`) when there is no `Theme` ancestor.
`ThemeData()` captures `defaultTargetPlatform` at construction, so the
fallback's platform is frozen to whatever platform was in effect the
first time a theme-less widget called `Theme.of(context)`. Under a
`TargetPlatformVariant`, `debugDefaultTargetPlatformOverride` every
later theme-less subtree keeps that first platform's selection controls,
toolbar, cursor and other adaptive behavior. In a shuffled test run this
makes theme-less tests depend on test order: with seed 20261008,
text_field_test.dart fails 45 tests that pass in declaration order.

**Fix:** Rebuild the cached fallback whenever its platform no longer
matches `defaultTargetPlatform`, so a theme-less subtree adapts to the
current platform the same way a `MaterialApp` does. `Theme`s built with
an explicit platform are unaffected; only the fallback is compared. Adds
a regression test in `theme_test.dart` that fails on master for every
non-android variant.

With the fallback following the variant, the two handle-drag tests in
`text_field_test.dart` fail because they had only ever run against
Android handles: desktop platforms draw no selection handles, and the
Cupertino start handle sits above the line. These tests have been
adjusted; restricted to the platforms that draw handles, the iOS start
handle is grabbed where it actually is, the Fuchsia toolbar that covers
the handles is hidden, and a test asserts that desktop platforms draw no
handles. The orphaned no-shuffle tag is removed.

**Other alternatives considered:**
- `text_field_test.dart` only fix, tests explicitly install their own
theme so `Theme.of(context)` does not return leaky fallback. Decided
against this since it wouldn't fix this leak for all users as reported
in flutter#83788

**This will still need a follow up PR that does the same change in
`material_ui`**

Fixes flutter#83788
Fixes flutter#194008

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

…lutter#194008)

Reverts flutter#193550 and restores the no-shuffle tag definition in packages/flutter/dart_test.yaml on flutter-3.50-candidate.0, where Theme._kFallbackTheme platform caching (flutter#194008, fixed after branch cut in flutter#194256) causes 56 tests in text_field_test.dart to fail under shuffled test seeds such as 20261103.

This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/blob/main/docs/releases/Flutter-Cherrypick-Process.md#automatically-creates-a-cherry-pick-request)

Cherry-pick of #193528 (already cherry-picked to `3.47.9` stable in #195015, ensuring `3.50` does not regress public API shipped in `3.47.9`).

Also includes `e58c176fe4` / `7d0aca2908` (`+12/-1`, test-only) restoring the `no-shuffle` tag on `packages/flutter/test/material/text_field_test.dart` (reverting #193550 and restoring `no-shuffle` in `packages/flutter/dart_test.yaml`) because `3.50` branched before #194256 fixed #194008 (`Theme._kFallbackTheme` platform caching across shuffled `TargetPlatformVariant` tests).

### Issue Link:
#193527
#194008

### Impact Description:
`ContextMenuButtonKind` and `ContextMenuButtonItem.kind` shipped in Flutter `3.47.9` stable (`#195015`) after `flutter-3.50-candidate.0` branched. Without cherry-picking `#193528` into `flutter-3.50-candidate.0`, packages and apps adopting `ContextMenuButtonKind` on `3.47.9` stable (including `cupertino_ui` and `material_ui` for iOS Translate context menu support in `#180021`) would fail to compile on `3.50.0`. The change itself is purely additive and does not deprecate `ContextMenuButtonType` or alter existing behavior. Restoring `no-shuffle` on `text_field_test.dart` unblocks `framework_tests_libraries` and `Mac framework_tests_impeller` on `flutter-3.50-candidate.0` under daily shuffled test seeds.

### Changelog Description:
- [flutter/193527](#193527) When building custom text selection menus, packages can't add new actions such as Translate without breaking exhaustive switches on `ContextMenuButtonType`; adds the extensible `ContextMenuButtonKind`.

### Workaround:
No.

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
Run `flutter test test/widgets/context_menu_button_item_test.dart test/widgets/editable_text_test.dart test/widgets/selectable_region_test.dart test/widgets/system_context_menu_test.dart` and `flutter test --test-randomize-ordering-seed=20261103 --fatal-warnings test/material/text_field_test.dart` to verify `ContextMenuButtonKind` backwards compatibility and `text_field_test.dart` execution under shuffled seeds.