Refactor combo box with browse

[cj-radcliff]

This PR replaces a deprecated ComboBoxWithBrowse button and eliminates a verification error. It was vibe coded with Gemini.

This change removes the deprecated ComboBoxWithBrowse button from the dart plugin. It exchanges it with a standard ComboBox. The functionality has been tested and is confirmed to be identical.

New Component: DartComboBoxWithBrowseButton.java is included in the branch.
EDT Safety: All three major UI areas (Remote Debug Configuration, SDK Settings, and Project Generator) are now using asynchronous, non-blocking ReadActions
for file system operations.
Build: The project compiles successfully with ./gradlew compileJava.
Scope: The changes are strictly limited to the UI refactoring and necessary threading fixes.
Summary of Changes

Core Component: Implemented DartComboBoxWithBrowseButton as a reusable, async-safe UI component.
Remote Debug Editor: Moved project indexing and searching to background threads.
SDK Settings: Refactored SDK validation and version loading to be non-blocking.
Project Generator: Implemented async SDK validation with result caching to satisfy the wizard's synchronous validation requirements without blocking the EDT.
The places to test:

Launch the IDE
Run the following command from the third_party directory:
1 ./gradlew runIde
(Note: Based on the build.gradle.kts, you can also specify a target version, e.g., ./gradlew runIde -Pide=IntelliJ -PideV=2024.2)

Verify the Changes
Once the IDE is running, you can find the modified UI components in these locations:
Dart Remote Debug Configuration (Smart Suggestions)

Open a Dart project or create a new one.
Go to Run -> Edit Configurations...
Click the + button and select Dart Remote Debug.
Look at the Dart project field.
Verification: In the combo-box-reset branch, this should be a ComboBox with a browse icon (folder icon) on the right. It should automatically suggest paths to any folders containing a pubspec.yaml file in your current workspace.
Dart SDK Settings (Centralized Setup)

Go to File -> Settings (or IntelliJ IDEA -> Settings on macOS).
Navigate to Languages & Frameworks -> Dart.
Look at the Dart SDK path field.
Verification: This field should now use the new ExtendableTextField implementation. When you click the browse button or change the path, the version label below it should update automatically using the centralized logic in DartSdkUtil.
New Project Wizard

Go to File -> New -> Project...
Select Dart from the left sidebar.
Check the Dart SDK path field here as well.

[gemini-code-assist]

Code Review

This pull request replaces the deprecated ComboboxWithBrowseButton with a custom BasicComboBoxWithBrowseButton across several UI components and refactors heavy operations, such as SDK validation and project searching, to run asynchronously using non-blocking ReadAction. These changes improve UI responsiveness and resource management by properly registering disposables for listeners. However, a regression was identified in the DartGeneratorPeer where the removal of a state listener prevents the wizard from updating the enabled state of navigation buttons; manual re-validation is recommended to ensure the wizard UI behaves correctly.

[gemini-code-assist]

By removing the addSettingsStateListener method, the project wizard no longer receives updates when the validation state changes (e.g., after the asynchronous SDK path validation completes). This can lead to the 'Next'/'Finish' button having an incorrect enabled state.

To fix this, you need to trigger validation manually. The SettingsStep passed to buildUI provides a checkValid() method for this purpose.

1. Store the SettingsStep in a field:

   private @Nullable SettingsStep mySettingsStep;
   
   @Override
   public void buildUI(final @NotNull SettingsStep settingsStep) {
     mySettingsStep = settingsStep;
     // ... existing code
   }

2. Trigger re-validation in onSdkPathChanged after the async validation completes:

   // inside .finishOnUiThread() in onSdkPathChanged()
   mySdkPathValidationError = errorMessage;
   if (mySettingsStep != null) {
       mySettingsStep.checkValid();
   }
   // ... existing code

This will ensure the wizard UI updates correctly.

References

1. This is a logical bug in the UI. According to the style guide, logical bugs are considered [MUST-FIX].

[cj-radcliff]

@pq fyi, we decided to remove the checkValid stuff. Happy to chase this down if you think Gemini is right here.

[pq]

Thanks!