feat: making the McpAsyncToolset constructor protected for subclassing

PiperOrigin-RevId: 899218434

Author: google-genai-bot

core/src/main/java/com/google/adk/tools/mcp/McpAsyncToolset.java

@@ -127,7 +127,7 @@ public McpAsyncToolset build() {
    * @param objectMapper An ObjectMapper instance for parsing schemas.
    * @param toolFilter Either a ToolPredicate or a List of tool names.
    */
-  McpAsyncToolset(
+  protected McpAsyncToolset(
       McpSessionManager mcpSessionManager, ObjectMapper objectMapper, @Nullable Object toolFilter) {
     Objects.requireNonNull(mcpSessionManager);
     Objects.requireNonNull(objectMapper);