chore: Add comprehensive test cases for RequestValidator to improve coverage and debugging #902

[manisha1997]

The RequestValidator test suite previously had only 11 basic test cases, making it difficult for developers to diagnose validation issues and understand expected behavior in edge cases. This PR significantly expands test coverage by adding 17 new comprehensive test cases.

What's Changed

New test coverage includes:

• Empty and null parameter handling - Tests validation with empty maps, null parameters, and missing form data
• Query parameter scenarios - URLs with and without query parameters, including URL-encoded values and non-Twilio parameters
• Edge case data handling - Special characters, unicode, very long values (1000+ chars), empty/null parameter values
• Behavioral verification - Parameter key case sensitivity, alphabetical sorting, international characters in URLs
• Error conditions - Invalid, null, and empty signatures to ensure proper failure modes

Example of improved debugging capabilities:

// Before: Only basic validation test
@Test
public void testValidate() {
    Assert.assertTrue("Request does not match provided signature", 
        validator.validate(url, params, signature));
}

// After: Comprehensive edge case coverage
@Test
public void testValidateWithSpecialCharactersInParams() {
    Map<String, String> specialParams = new HashMap<>();
    specialParams.put("Message", "Hello & goodbye! @#$%^*()");
    specialParams.put("Special", "unicode: ñáéíóú");
    
    String expectedSignature = "dCPiR4WtQ6QFN6pJh81CtlCcWLQ=";
    boolean isValid = validator.validate(url, specialParams, expectedSignature);
    
    Assert.assertTrue("Validation should succeed with special characters", isValid);
}

Impact

• Test count increased from 11 to 28 - More than doubled test coverage
• Better developer experience - Clear examples of expected behavior across various scenarios
• Improved reliability - Edge cases that could cause real-world validation failures are now tested
• Enhanced security - Comprehensive testing of the security-critical RequestValidator component

All new test cases use correctly calculated signatures and follow existing project conventions. The expanded test suite makes RequestValidator much more usable and debuggable for developers integrating Twilio webhooks.

Fixes #645.

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[shrutiburman]

what if we dont type-cast null, we should expect a NPE?