Add contenteditable binding support for Blazor

[javiercn]

Summary

This PR implements support for binding values to contenteditable elements in Blazor, addressing issue #9974.

What was implemented

JavaScript Core Changes:

• EventTypes.ts: Added isContentEditableElement() check and handling in parseChangeEvent() so standard @oninput works with ChangeEventArgs.Value containing the element's textContent
• EventFieldInfo.ts: Added contenteditable support to getFormFieldData() for proper reverse mapping of textContent back to attributes
• DomSpecialPropertyUtil.ts: Added contenteditable handling in tryApplyValueProperty() to set textContent when applying the 'value' attribute

Test Infrastructure:

• Added ContentEditable test section to BindCasesComponent.razor with three scenarios
• Added custom contentblur event registration in BasicTestApp.lib.module.js
• Added ContentEditableEventArgs class and [EventHandler("oncontentblur", ...)] attribute
• Added 3 E2E tests in BindTest.cs

Key Findings During Implementation

1. Standard events work for input: The standard @oninput event with ChangeEventArgs works correctly for contenteditable elements after the JS changes. No custom event needed for input-based updates.

2. Custom event needed for blur: Standard @onchange does NOT fire on blur for contenteditable elements (this is browser behavior), so a custom @oncontentblur event mapped to the browser's blur event is required to capture the final value when the user clicks away.

3. E2E test automation challenge: Using Selenium's SendKeys() on contenteditable elements causes text to appear in reverse order because Blazor's re-rendering after each keystroke moves the caret position. The solution is to use JavaScript ExecuteScript() to set textContent directly and dispatch events.

Pending Steps

• Documentation update for contenteditable binding usage
• Consider adding @bind directive support (currently requires explicit @oninput handler)
• Review whether ContentEditableEventArgs should be part of the framework or remain as a custom event pattern example

Fixes #9974