refactor: reorganize samples structure and add Maven exec plugin

- Add exec-maven-plugin configuration to samples/pom.xml for easier example execution
- Move example files from ai.z.openapi.samples package to java/ai/z/openapi/samples directory
- Relocate all example classes including AgentExample, AudioSpeechExample, ChatCompletionExample, etc.
- No functional changes to example code, only directory structure reorganization

Author: tomsun28

samples/pom.xml

@@ -17,4 +17,17 @@
             <version>${revision}</version>
         </dependency>
     </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.codehaus.mojo</groupId>
+                <artifactId>exec-maven-plugin</artifactId>
+                <version>3.1.0</version>
+                <configuration>
+                    <classpathScope>compile</classpathScope>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
 </project>