Add #[RunTestsInSeparateProcesses] attribute to all kernel test classes
## Description
Drupal 11.3 deprecates kernel test classes that do not declare the PHPUnit `#[RunTestsInSeparateProcesses]` attribute. From Drupal 12.0, missing the attribute will throw an exception ([change record](https://www.drupal.org/node/3548485)).
The ai_context module currently has **4 of 39** kernel test classes with the attribute:
- `AiContextChildrenServiceTest`
- `AiContextItemFormGlobalCountTest`
- `AiContextTextFieldMigrationTest`
- `AiContextUpdate10010Test`
The remaining **35** kernel test classes need:
```php
use PHPUnit\Framework\Attributes\RunTestsInSeparateProcesses;
#[RunTestsInSeparateProcesses]
final class ExampleKernelTest extends KernelTestBase {
```
**Notes**
- Low urgency for 11.x; required before Drupal 12 compatibility.
## Acceptance criteria
- Every class under `tests/src/Kernel/` that extends `KernelTestBase` has `#[RunTestsInSeparateProcesses]`.
- Full kernel test suite runs without the “Kernel test classes must specify the #[RunTestsInSeparateProcesses] attribute” deprecation.
- No test behavior regressions.
## AI usage
- [x] AI assisted issue
issue