Enable Safari dart2wasm/skwasm CI suites on macOS 26.6 - #193681

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

#193833, #193834

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

CI test-only change to migrate tests to macOS 26. This is a partial cherry-pick of #193681, only including the parts required for macOS 26.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

N/A - CI test-only change

### Workaround:
Is there a workaround for this issue?

Rerun test

### 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:
What are the steps to validate that this fix works?

Run in CI

This switches the Safari Web engine CI suites (`Mac run safari suites`)
to run on `Mac-26.6` LUCI bots (matching the `Mac-26.6` dimension
convention in flutter#193317) and enables the 4 WasmGC / Skwasm Safari suites
now that Safari 26 (with Wasm `js-string` builtins) ships on macOS
26.6.2 bots (flutter#192559 / flutter#192572):

- `safari-dart2wasm-canvaskit-engine`
- `safari-dart2wasm-skwasm-ui`
- `safari-coi-dart2wasm-skwasm-ui`
- `safari-coi-dart2wasm-skwasm-skwasm`

### Safari 26 & WebDriver Focus Test Updates

Running `Mac run safari suites` on `Mac-26.6` exposed two Safari 26 CSS
`outline` serialization differences and one test that depended on
`Safari.app` holding macOS WindowServer foreground focus under
`safaridriver`:

1. **`text_editing_test.dart` (`text editing styles invisible
element`)**: Safari 26 serializes inline `outline: none` on
`input.style.outline` as `'medium'` (matching `input.style.border`
immediately below it).
2. **`view_embedder/style_manager_test.dart` (`attachGlobalStyles hides
the outline when focused`)**: Safari 26 serializes
`getComputedStyle(element).outline` in canonical CSSOM order (`'0px none
rgb(0, 0, 0)'` instead of `'rgb(0, 0, 0) none 0px'`). Additionally, in
WebKit `:focus` only matches when `domDocument.hasFocus()` is `true`;
when `Safari.app` is not the frontmost macOS WindowServer app
(`!domDocument.hasFocus()`), WebKit returns the unfocused initial
outline (`'3px none rgb(0, 0, 0)'`).
3. **`platform_dispatcher/view_focus_binding.dart` &
`view_focus_binding_test.dart` (`works even if focus is changed in the
middle of a blur call`)**: Use `_documentHasFocus` (introduced in
flutter#190942 for `debugViewFocusDocumentHasFocusOverride`) in
`ViewFocusBinding._handleFocusout` (`wasFocusInvoked`) and set
`debugViewFocusDocumentHasFocusOverride = true` in the test so it does
not flake when the `safaridriver` window is unfocused in WindowServer.

Fixes flutter#193833
Fixes flutter#193834
Part of flutter#192559
Part of flutter#192572

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

flutter/flutter#193833, flutter/flutter#193834

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

CI test-only change to migrate tests to macOS 26. This is a partial cherry-pick of flutter/flutter#193681, only including the parts required for macOS 26.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

N/A - CI test-only change

### Workaround:
Is there a workaround for this issue?

Rerun test

### 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:
What are the steps to validate that this fix works?

Run in CI

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

flutter/flutter#193833, flutter/flutter#193834

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

CI test-only change to migrate tests to macOS 26. This is a partial cherry-pick of flutter/flutter#193681, only including the parts required for macOS 26.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

N/A - CI test-only change

### Workaround:
Is there a workaround for this issue?

Rerun test

### 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:
What are the steps to validate that this fix works?

Run in CI

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

flutter/flutter#193833, flutter/flutter#193834

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
or the shipping of production apps (the app crashes on launch).
This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.

CI test-only change to migrate tests to macOS 26. This is a partial cherry-pick of flutter/flutter#193681, only including the parts required for macOS 26.

### Changelog Description:
Explain this cherry pick:
* In one line that is accessible to most Flutter developers.
* That describes the state prior to the fix.
* That includes which platforms are impacted.
See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.

N/A - CI test-only change

### Workaround:
Is there a workaround for this issue?

Rerun test

### 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:
What are the steps to validate that this fix works?

Run in CI