Add comprehensive unit tests for CSS parser core classes

[Copilot]

The CSS parser had incomplete test coverage, particularly in core parser classes, DOM implementations, selector logic, and error handling paths.

Changes

Added 105 new tests (+3% total) across critical areas:

Parser Layer

• CssCharStreamTest (19 tests): Buffer management, line/column tracking, wrap-around scenarios, backup operations
• CSSErrorHandlingTest (15 tests): Malformed CSS recovery, invalid unicode escapes, buffer overflow, edge cases

DOM Layer

• AbstractCSSRuleImplTest (11 tests): Parent relationship handling, circular reference prevention in equals/hashCode
• Enhanced PropteryTest (+6 tests): Null handling, equality edge cases

Selector Layer

• ComplexSelectorTest (15 tests): Combinator chains (>, +, ~), pseudo-classes, deeply nested selectors
• Enhanced DescendantSelectorImplTest (+5 tests): Null ancestors, complex nesting
• Enhanced SelectorSpecificityTest (+9 tests): :not()/:is()/:where() edge cases, multiple IDs

Condition & Media

• Enhanced PseudoClassConditionTest (+5 tests): Double colon (::) vs single colon (:) handling
• Enhanced MediaQueryTest (+9 tests): only/not modifiers, property combinations

Example Coverage

// CssCharStream now has comprehensive buffer wrap-around testing
@Test
public void getImageWrapAround() throws Exception {
    StringBuilder input = new StringBuilder();
    for (int i = 0; i < 3000; i++) {
        input.append((char) ('a' + (i % 26)));
    }
    
    CssCharStream stream = new CssCharStream(new StringReader(input.toString()), 1, 1, 100);
    stream.beginToken();
    for (int i = 0; i < 49; i++) {
        stream.readChar();
    }
    
    String image = stream.getImage();
    assertEquals(50, image.length());
}

// Error handling tests verify parser recovers from malformed CSS
@Test
public void malformedSelector() throws Exception {
    String css = "p { color: red; }\n"
               + "div# { font-size: 12px; }\n"  // malformed
               + "span { margin: 10px; }";
    
    CSSStyleSheetImpl sheet = parser.parseStyleSheet(new InputSource(new StringReader(css)), null);
    assertNotNull(sheet);  // Parser recovers and continues
}

All 730 tests pass. Zero security vulnerabilities detected.

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Original prompt

Problem Statement

The HtmlUnit CSS parser project currently has incomplete test coverage. This PR aims to add comprehensive unit tests to achieve 100% test coverage for the parser, focusing on:

Areas Requiring Additional Test Coverage

1. Parser Classes - Missing comprehensive tests for:

   • org.htmlunit.cssparser.parser.CssCharStream - Character stream handling, buffer management, line/column tracking
   • org.htmlunit.cssparser.parser.javacc.CSS3Parser - Core parser functionality, error handling, edge cases
   • Parser error recovery and malformed CSS handling
   • Unicode escape sequence processing
   • Token boundary conditions

2. DOM Classes - Incomplete coverage for:

   • org.htmlunit.cssparser.dom.Property - All setter/getter methods, equals/hashCode edge cases
   • org.htmlunit.cssparser.dom.CSSValueImpl - All value types and conversions
   • org.htmlunit.cssparser.dom.AbstractCSSRuleImpl - Parent relationship handling
   • Various CSS rule implementations

3. Selector Classes - Missing tests for:

   • org.htmlunit.cssparser.parser.selector.DescendantSelector - Complex descendant relationships
   • org.htmlunit.cssparser.parser.selector.SelectorSpecificity - Edge cases in specificity calculation
   • Complex selector combinations
   • Pseudo-element and pseudo-class edge cases

4. Condition Classes - Gaps in:

   • org.htmlunit.cssparser.parser.condition.PseudoClassCondition - Double colon handling
   • org.htmlunit.cssparser.parser.condition.SuffixAttributeCondition - Case sensitivity
   • All attribute condition variants (prefix, substring, begin-hyphen, one-of)
   • Complex condition combinations

5. Media Query Support:

   • org.htmlunit.cssparser.parser.media.MediaQuery - All media types and features
   • Complex media query combinations
   • Media query error handling

6. Error Handling & Edge Cases:

   • Malformed CSS recovery
   • Buffer overflow conditions
   • Invalid character handling
   • Null/empty input handling
   • Exception paths and error reporting

Test Coverage Goals

Create comprehensive test suites that cover:

• All public methods in core parser classes
• Edge cases and boundary conditions
• Error handling and recovery
• Unicode and special character handling
• Complex CSS syntax combinations
• Null safety and defensive programming
• Equals/hashCode contracts
• Serialization/deserialization if applicable
• Thread safety where relevant

Expected Outcomes

• Achieve 100% line coverage for parser core classes
• Achieve 100% branch coverage for critical parsing logic
• Comprehensive test documentation
• Tests that serve as usage examples
• Improved code quality through testing edge cases found during test development

Implementation Guidelines

• Use JUnit 5 (Jupiter) as seen in existing tests
• Follow existing test patterns and naming conventions
• Include both positive and negative test cases
• Test error messages and exception types
• Add tests for CSS3 specific features
• Ensure tests are isolated and don't depend on execution order
• Add JavaDoc comments explaining complex test scenarios

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