Seal GeoIp2Exception hierarchy

Make GeoIp2Exception a sealed class with an explicit permits clause
listing all allowed subclasses. This provides:

- Explicit API stability guarantees - new exception types cannot be
  added externally
- Compiler-enforced exhaustiveness checking for pattern matching in
  future Java versions
- Clear documentation of the complete exception hierarchy

All subclasses (AddressNotFoundException, AuthenticationException,
InvalidRequestException, OutOfQueriesException, and
PermissionRequiredException) were already marked as final, making them
compatible with the sealed parent.

Note: HttpException extends IOException, not GeoIp2Exception, so it
remains outside this sealed hierarchy.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: oschwald

src/main/java/com/maxmind/geoip2/exception/GeoIp2Exception.java

@@ -4,7 +4,12 @@
  * This class represents a generic GeoIP2 error. All other exceptions thrown by
  * the GeoIP2 API subclass this exception
  */
-public class GeoIp2Exception extends Exception {
+public sealed class GeoIp2Exception extends Exception
+    permits AddressNotFoundException,
+            AuthenticationException,
+            InvalidRequestException,
+            OutOfQueriesException,
+            PermissionRequiredException {
 
 
     /**