feat(sdk): add linter and fix linter findings

core/src/main/java/cloud/stackit/sdk/core/KeyFlowAuthenticator.java

@@ -4,6 +4,7 @@
 import cloud.stackit.sdk.core.config.CoreConfiguration;
 import cloud.stackit.sdk.core.config.EnvironmentVariables;
 import cloud.stackit.sdk.core.exception.ApiException;
+import cloud.stackit.sdk.core.exception.AuthenticationException;
 import cloud.stackit.sdk.core.model.ServiceAccountKey;
 import cloud.stackit.sdk.core.utils.Utils;
 import com.auth0.jwt.JWT;
@@ -132,7 +133,7 @@ public Request authenticate(Route route, @NotNull Response response) throws IOEx
 		try {
 			accessToken = getAccessToken();
 		} catch (ApiException | InvalidKeySpecException e) {
-			throw new IllegalStateException(e);
+			throw new AuthenticationException("Failed to obtain access token", e);
 		}
 
 		// Return a new request with the refreshed token
@@ -215,7 +216,7 @@ protected void createAccessToken() throws InvalidKeySpecException, IOException,
 			try {
 				assertion = generateSelfSignedJWT();
 			} catch (NoSuchAlgorithmException e) {
-				throw new IllegalStateException(
+				throw new AuthenticationException(
 						"could not find required algorithm for jwt signing. This should not happen and should be reported on https://github.com/stackitcloud/stackit-sdk-java/issues",
 						e);
 			}

core/src/main/java/cloud/stackit/sdk/core/KeyFlowInterceptor.java

@@ -1,6 +1,7 @@
 package cloud.stackit.sdk.core;
 
 import cloud.stackit.sdk.core.exception.ApiException;
+import cloud.stackit.sdk.core.exception.AuthenticationException;
 import java.io.IOException;
 import java.security.spec.InvalidKeySpecException;
 import okhttp3.Interceptor;
@@ -37,7 +38,7 @@ public Response intercept(Chain chain) throws IOException {
 		} catch (InvalidKeySpecException | ApiException e) {
 			// try-catch required, because ApiException can not be thrown in the implementation
 			// of Interceptor.intercept(Chain chain)
-			throw new IllegalStateException(e);
+			throw new AuthenticationException("Failed to obtain access token for request authentication", e);
 		}
 
 		Request authenticatedRequest =

core/src/main/java/cloud/stackit/sdk/core/exception/AuthenticationException.java

@@ -0,0 +1,37 @@
+package cloud.stackit.sdk.core.exception;
+
+/**
+ * Exception thrown when authentication operations fail.
+ * This includes token generation, refresh, and validation failures.
+ */
+public class AuthenticationException extends RuntimeException {
+	private static final long serialVersionUID = 1L;
+
+	/**
+	 * Constructs a new AuthenticationException with the specified detail message.
+	 *
+	 * @param message the detail message
+	 */
+	public AuthenticationException(String message) {
+		super(message);
+	}
+
+	/**
+	 * Constructs a new AuthenticationException with the specified detail message and cause.
+	 *
+	 * @param message the detail message
+	 * @param cause the cause of this exception
+	 */
+	public AuthenticationException(String message, Throwable cause) {
+		super(message, cause);
+	}
+
+	/**
+	 * Constructs a new AuthenticationException with the specified cause.
+	 *
+	 * @param cause the cause of this exception
+	 */
+	public AuthenticationException(Throwable cause) {
+		super(cause);
+	}
+}