Fix response parsing in shaded builds and modernize test environment (#41, #333)

[maheshshinde9100]

Problem: Users reported ClassCastExceptions (Issue #41 and #333) when the SDK was re-packaged or "shaded" into a different namespace. This happened because ApiClient.java had a hardcoded "com.razorpay." string, causing reflection to fail when looking up entity classes in different packages. Additionally, the test suite failed on modern Java versions (JDK 17+) due to legacy Mockito configurations.

Changes:

1. Dynamic Package Resolution: Modified ApiClient.java to use getClass().getPackage().getName() instead of hardcoded strings. This ensures the SDK correctly identifies its own package at runtime, regardless of shading.
2. Build Modernization: Updated Mockito to 4.11.0 and Jacoco to 0.8.11 to support Java 21+. Configured maven-surefire-plugin with -XX:+EnableDynamicAgentLoading and -Dnet.bytebuddy.experimental=true to allow mocking on newer JVMs.
3. Test Refactoring: Updated BaseTest.java to replace the deprecated anyObject() matcher with any(). Migrated initMocks() to openMocks() for Mockito 4 compatibility.
4. Cleanup: Removed duplicate org.json dependency in pom.xml.

Impact:

• Fixes the "Unable to parse response" error for all developers using custom package structures or build shading.
• Allows contributors to run the full test suite on modern development environments (JDK 8 through 21).

Verification:

• Successfully executed mvn clean compile.
• Passed all 170 unit tests using mvn test on Java 21.