fix(deploy): add --agent_module flag to adk deploy agent_engine

[RudrenduPaul]

Fixes #5192

adk deploy agent_engine auto-generates agent_engine_app.py with a hardcoded from .agent import <object>. This assumes the user's entry point is always named agent.py. Any other filename (e.g. core.py, adk_agent.py) causes a ModuleNotFoundError at deployment time — silently, in the container build on Vertex AI, where it is hard to debug.

Changes

• cli_deploy.py — Add agent_module: Optional[str] = None parameter to to_agent_engine(). Defaults to 'agent' for backward compatibility. Pass it through to _AGENT_ENGINE_APP_TEMPLATE and _validate_agent_import.
• cli_deploy.py — Update _validate_agent_import to check {agent_module}.py instead of hardcoded agent.py, and improve the error message to reference --agent_module.
• cli_tools_click.py — Add --agent_module CLI option to adk deploy agent_engine. Pass it through to to_agent_engine().

Usage

# Default behaviour unchanged
adk deploy agent_engine --project=my-project my_agent/

# Custom entry point
adk deploy agent_engine --project=my-project --agent_module=core my_agent/
adk deploy agent_engine --project=my-project --agent_module=adk_agent my_agent/

Backward compatibility

Default is agent, so all existing deployments continue to work without any changes.

Testing

Verified the generated agent_engine_app.py contains from .core import root_agent when --agent_module=core is passed, and from .agent import root_agent when the flag is omitted.

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[RudrenduPaul]

I have read the CLA Document, and I hereby sign the CLA.

[RudrenduPaul]

I have read the CLA Document and I hereby sign the CLA.

[RudrenduPaul]

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