feat: Add process LLM request API to BaseToolset

Author: r4inee

core/src/main/java/com/google/adk/flows/llmflows/BaseLlmFlow.java

@@ -38,6 +38,8 @@
 import com.google.adk.models.LlmRequest;
 import com.google.adk.models.LlmResponse;
 import com.google.adk.telemetry.Tracing;
+import com.google.adk.tools.BaseTool;
+import com.google.adk.tools.BaseToolset;
 import com.google.adk.tools.ToolContext;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
@@ -99,10 +101,24 @@ private Flowable<Event> preprocess(
     RequestProcessor toolsProcessor =
         (ctx, req) -> {
           LlmRequest.Builder builder = req.toBuilder();
-          return agent
-              .canonicalTools(new ReadonlyContext(ctx))
+          ReadonlyContext readonlyContext = new ReadonlyContext(ctx);
+          return Flowable.fromIterable(agent.toolsUnion())
               .concatMapCompletable(
-                  tool -> tool.processLlmRequest(builder, ToolContext.builder(ctx).build()))
+                  toolOrToolset -> {
+                    ToolContext toolContext = ToolContext.builder(ctx).build();
+                    if (toolOrToolset instanceof BaseToolset toolset) {
+                      return toolset
+                          .processLlmRequest(builder, toolContext)
+                          .andThen(
+                              toolset
+                                  .getTools(readonlyContext)
+                                  .concatMapCompletable(
+                                      tool -> tool.processLlmRequest(builder, toolContext)));
+                    } else if (toolOrToolset instanceof BaseTool tool) {
+                      return tool.processLlmRequest(builder, toolContext);
+                    }
+                    return Completable.complete();
+                  })
               .andThen(
                   Single.fromCallable(
                       () -> RequestProcessingResult.create(builder.build(), ImmutableList.of())));

core/src/main/java/com/google/adk/tools/BaseToolset.java

@@ -17,6 +17,8 @@
 package com.google.adk.tools;
 
 import com.google.adk.agents.ReadonlyContext;
+import com.google.adk.models.LlmRequest;
+import io.reactivex.rxjava3.core.Completable;
 import io.reactivex.rxjava3.core.Flowable;
 import java.util.List;
 import org.jspecify.annotations.Nullable;
@@ -32,6 +34,21 @@ public interface BaseToolset extends AutoCloseable {
    */
   Flowable<BaseTool> getTools(ReadonlyContext readonlyContext);
 
+  /**
+   * Called during LLM request preprocessing, before the toolset's individual tools are processed.
+   * Allows the toolset to modify the LLM request, e.g. to append system instructions.
+   *
+   * <p>The default implementation is a no-op.
+   *
+   * @param llmRequestBuilder The builder for the LLM request to be modified.
+   * @param toolContext The tool context providing access to session state and agent information.
+   * @return A {@link Completable} that completes when processing is done.
+   */
+  default Completable processLlmRequest(
+      LlmRequest.Builder llmRequestBuilder, ToolContext toolContext) {
+    return Completable.complete();
+  }
+
   /**
    * Performs cleanup and releases resources held by the toolset.
    *