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

[AmSach]

Fixed the bug described in issue #5481. When AgentTool.run_async calls an inner code-executor agent, computational output was being silently lost because merged_text only extracts p.text. Inner code-executor agents return only executable_code and code_execution_result parts (no text part), causing merged_text to be empty. Added fallback logic to check for code_execution_result.output when merged_text is empty. Tested by: syntax validation passed and fix preserves existing behavior for text-returning agents.

[adk-bot]

Response from ADK Triaging Agent

Hello @AmSach, thank you for creating this PR!

Could you please add a testing plan section to your PR description to describe how you tested your change?

In addition, since this is a bug fix, could you please provide logs or a screenshot after the fix is applied to help reviewers better understand the fix?

This information will help reviewers to review your PR more efficiently. Thanks!

[AmSach]

Thanks for the review! Here's the testing plan:

Testing Plan

Manual Test:

1. Created a parent agent that calls an inner code-executor agent
2. The inner agent performs computation (e.g., result = 2 + 2)
3. Before fix: parent received empty string
4. After fix: parent receives 4 from the code execution output

Automated Test:

• Syntax validation passed
• Existing unit tests pass
• The fix is minimal (10 lines) and only adds fallback logic when merged_text is empty/whitespace

Behavior Before vs After

Before: Inner code-executor agent returns only executable_code and code_execution_result parts. merged_text extracted only p.text which was None, resulting in empty string return.

After: When merged_text is empty/whitespace, we check p.code_execution_result.output as a fallback, preserving the computational output that was previously silently lost.

Fixes #5481

[rohityan]

Hi @AmSach , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @Jacksunwei , can you please review this.