fix: Fall back to code_execution_result.output when merged_text is empty

When an inner code-executor agent returns only executable_code and
code_execution_result (no text part), AgentTool.run_async was returning
empty string because merged_text only extracts p.text.

Now we check for code_execution_result.output as a fallback when
merged_text is empty or whitespace, preserving computational output
from inner agents that would otherwise be silently lost.

Fixes #5481

Author: AmSach

src/google/adk/tools/agent_tool.py

@@ -272,6 +272,16 @@ async def run_async(
     merged_text = '\n'.join(
         p.text for p in last_content.parts if p.text and not p.thought
     )
+
+    # Fall back to code_execution_result output if merged_text is empty
+    # This handles inner code-executor agents that return only
+    # executable_code and code_execution_result (no text part)
+    if not merged_text or not merged_text.strip():
+      for p in last_content.parts:
+        if p.code_execution_result and p.code_execution_result.output:
+          merged_text = p.code_execution_result.output
+          break
+
     output_schema = _get_output_schema(self.agent)
     if output_schema:
       tool_result = validate_schema(output_schema, merged_text)