Make UI.wrapperElement non-public by adding private field and public getter

[Copilot]

The wrapperElement field in the UI class was incorrectly declared as public, which violates encapsulation principles and exposes internal implementation details.

This PR addresses the issue by:

1. Making the field private: Changed public Element wrapperElement; to private Element wrapperElement;
2. Adding a public getter: Created public Element getWrapperElement() method with proper JavaDoc documentation
3. Updating all external usages: Modified external classes to use the getter method instead of direct field access

Changes Made

• UI.java: Field made private and getter method added
• UIInternalUpdater.java: Updated to use ui.getWrapperElement()
• BeforeLeaveEvent.java: Updated 4 usages to use the getter method
• JavaScriptBootstrapUITest.java: Updated 26 test usages to use the getter method

Benefits

• Proper encapsulation: The field is now private with controlled access
• API compatibility: External code can still access the wrapper element via the public getter
• Internal consistency: UI class internal usage remains unchanged
• Zero breaking changes: All existing functionality is preserved

The wrapperElement is used as a container for route components when client-side routing is enabled, and this refactoring ensures it follows proper object-oriented design principles while maintaining full backward compatibility.

Fixes #22008.

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[github-actions]

Format Checker Report

There are 2 files with format errors

• To see a complete report of formatting issues, download the differences artifact

• To fix the build, please run mvn spotless:apply in your branch and commit the changes.

• Optionally you might add the following line in your .git/hooks/pre-commit file:

  mvn spotless:apply
  

Here is the list of files with format issues in your PR:

flow-server/src/main/java/com/vaadin/flow/component/UI.java
flow-server/src/test/java/com/vaadin/flow/component/internal/JavaScriptBootstrapUITest.java

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test Results

1 306 files  ± 0  1 306 suites  ±0   1h 15m 5s ⏱️ - 1m 39s
9 171 tests ± 0  9 104 ✅ ± 0  67 💤 ±0  0 ❌ ±0 
9 598 runs   - 35  9 524 ✅  - 34  74 💤  - 1  0 ❌ ±0 

Results for commit baf1578. ± Comparison against base commit 61ea69f.

[platosha]

As per #22008 (comment), it would make sense to avoid public API exposure for this getter. Let's consider moving it to UIInternals and possibly making package-private or marked as internal API.

[mcollovati]

Superseeded by #23194