SLING-13257: Improve Test Coverage

[apelluru]

This PR significantly increases the unit-test coverage of the org.apache.sling.auth.core bundle from roughly 43% to 90.79% instruction coverage, without any changes to production code. It adds new test classes and extends existing ones to exercise previously-untested paths across the SPI, impl, and engine-integration packages.

What changed

• No production/source changes — test-only additions.
• Extended existing test classes: AuthUtilTest, PathBasedHolderTest, SlingAuthenticatorTest.
• Added new test classes covering previously-untested components, including:
• impl: AuthenticationHandlerHolderTest, AuthenticationHandlerWrapperTest, AuthenticationRequirementHolderTest, AuthenticatorWebConsolePluginTest, HttpBasicAuthenticationHandlerTest, LoginServletTest, LogoutServletTest
• impl/engine: engine-integration wrapper tests
• spi: AbstractAuthenticationHandlerTest, AbstractAuthenticationFormServletTest, AbstractJakartaAuthenticationFormServletTest, DefaultAuthenticationFeedbackHandlerTest, DefaultJakartaAuthenticationFeedbackHandlerTest
• Covered both the legacy javax.servlet and Jakarta EE (jakarta.servlet) code paths where parallel variants exist.
• Exercised key edge cases: redirect validation (isRedirectValid), login-failure handling (handleLoginFailure, including TooManySessionsException and generic-error branches), handleSecurity flows, IOException error paths, and referer/URL validation.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud