refactor: make AgentStaticLoader public

This is needed to start an application with custom Spring configuration. AgentStaticLoader will be able to be instantiated as a bean.

PiperOrigin-RevId: 835220523

Author: google-genai-bot

dev/src/main/java/com/google/adk/web/AgentStaticLoader.java

@@ -37,7 +37,7 @@
  * <p>This class is not a Spring component by itself - instances are created programmatically and
  * then registered as beans via factory methods.
  */
-class AgentStaticLoader implements AgentLoader {
+public class AgentStaticLoader implements AgentLoader {
 
   private final ImmutableMap<String, BaseAgent> agents;