Deploy yaml configured agent to agent_engine

[furoTmark]

Describe the bug
I can create an run agents just by configuring a yaml file.
However when I want to deploy them it fails every time.

To Reproduce
Create a new folder with a minimal agent:

root_agent.yaml:

name: test
model: gemini-2.5-flash
agent_class: LlmAgent
instruction: You are the root agent that coordinates other agents.
sub_agents: []
tools: []

Run: adk deploy agent_engine .... test_agent

Expected behavior
Succesfull deploy.

Screenshots
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Desktop (please complete the following information):

• OS: Windows
• Python version(python -V): 3.12
• ADK version(pip show google-adk): 1.21

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-2.0-flash

Additional context
Add any other context about the problem here.

[wuliang229]

I can reproduce the issue. Working on a fix.