Implement HTML-AAM §4.1 compliant accessible name computation

[Copilot]

This PR reimplements accessible name computation to align with the HTML Accessibility API Mappings (HTML-AAM) specification §4.1 "Accessible Name Computations By HTML Element."

Problem

Previously, Markuplint used the dom-accessibility-api library for accessible name computations across all elements. However, according to HTML-AAM §4.1, certain HTML elements should compute their accessible names only when they possess aria-label or aria-labelledby attributes. Without these attributes, each element should follow specific conditions to determine their accessible names.

Solution

Conditional Application of accname Library

• Apply the dom-accessibility-api library exclusively to elements that have:
  • aria-label attribute
  • aria-labelledby attribute
  • role attribute (explicit ARIA roles)
  • style attribute (for proper visibility handling)

Element-Specific Accessible Name Logic

Implement custom logic for elements without the above attributes, adhering to HTML-AAM §4.1:

• <img>: Use the alt attribute, fallback to title
• <input>: Derive name from associated <label> elements or fallback to placeholder
• <textarea> and <select>: Use associated <label> elements
• <button>: Use the element's text content
• <a> and headings (<h1>-<h6>): Use text content
• <table>: Use <caption> element
• <fieldset>: Use <legend> element
• Other elements: Return empty string unless they have ARIA attributes

Key Implementation Details

// Before: Always used dom-accessibility-api
const name = computeAccessibleName(el);

// After: Conditional based on HTML-AAM §4.1
if (hasAriaNameAttributes(el)) {
    return computeAccessibleName(el); // Use accname library
}
return getElementSpecificAccessibleName(el); // Use element-specific logic

The implementation properly handles:

• Label association for form elements (searches both element root and document scope)
• Hidden elements (aria-hidden, hidden attribute, CSS styles)
• Nested text content extraction while excluding hidden elements
• Backward compatibility with existing behavior

Benefits

• ✅ Standards Compliance: Accessible name computation now aligns with HTML-AAM specification
• ✅ Improved Accuracy: More precise accessibility validations by considering element-specific naming conditions
• ✅ Performance Optimization: Reduced unnecessary computations by applying accname library only when relevant attributes are present

Testing

Added comprehensive test suite covering:

• Elements with aria-label/aria-labelledby (should use accname library)
• Element-specific logic for all supported HTML elements
• Label association scenarios for form controls
• Hidden element handling
• Edge cases and backward compatibility

All existing tests continue to pass, ensuring no regressions.

Fixes #2437.

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[YusukeHirao]

@copilot

Could you include https://github.com/markuplint/markuplint/issues/1478 as well?

[Copilot]

@copilot

Could you include https://github.com/markuplint/markuplint/issues/1478 as well? ...

I've included issue #1478 by implementing a new rule no-conflicting-accessible-name that detects when multiple accessible name sources are provided for an element (e.g., both <label> and aria-labelledby). The rule follows the same pattern as the example you provided and helps prevent developer confusion about which accessible name will be used. See commit c0c0ba6.