Add read-only assertions via HasReadOnlyElement mixin (#113)

[jcgueriaud1]

Fixes #113.

Problem

Asserting a field's read-only state required a raw property check:

TextFieldElement orderId = TextFieldElement.getByLabel(page, "Order Id");
assertEquals(Boolean.TRUE, orderId.getProperty("readonly"));

Change

Introduces a shared HasReadOnlyElement mixin (following the existing HasEnabledElement pattern) so fields expose first-class read-only helpers:

• isReadOnly(), isReadOnly(boolean)
• assertReadOnly(), assertReadOnly(boolean), assertNotReadOnly()

The issue's example becomes:

TextFieldElement orderId = TextFieldElement.getByLabel(page, "Order Id");
orderId.assertReadOnly();

Details

• TextFieldElement implements the mixin — inherited automatically by PasswordFieldElement, EmailFieldElement, and TextAreaElement.
• ComboBoxElement and MultiSelectComboBoxElement had duplicated read-only methods; they now use the shared mixin (behavior unchanged).
• Added a "Read-only Example" (with a toggle button) to TextFieldView and a testReadOnly IT.

Testing

• mvn test-compile passes.
• New TextFieldViewIT#testReadOnly passes.
• Existing ComboBoxViewIT#testReadOnly / MultiSelectComboBoxViewIT#testReadOnly still pass after the refactor.

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