feat(workflow): Support loading workflows from YAML configurations

[baptmont]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: N/A
• Related: N/A

2. Or, if no issue exists, describe the change:

Problem:
Workflows and workflow-specific node types (JoinNode, FunctionNode, ToolNode) defined in YAML config files could not be loaded or executed because:

1. The class resolution utilities lacked support for identifying and loading the Workflow class and workflow-specific nodes.
2. References to nested sub-agents and sub-nodes within workflow configuration files were parsed as raw strings rather than recursively resolved and loaded.

Solution:

1. Implemented the Workflow.from_config method to instantiate declarative workflow graphs from configuration inputs.
2. Added resolution support for Workflow, JoinNode, FunctionNode, and ToolNode in _resolve_node_class.
3. Implemented path-aware recursive resolution of nested sub-agent and node .yaml configurations during parsing.

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Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Summary of configuration-related test execution results:

• Agent configuration tests: uv run pytest tests/unittests/agents/test_agent_config.py
36 passed

Manual End-to-End (E2E) Tests:
Verified the correctness of workflow config loading by instantiating complex multi-agent graphs from declarative configurations in test_agent_config.py.

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Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

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Additional context

N/A

[rohityan]

Hi @baptmont , 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 @DeanChensj , can you please review this.

[boyangsvl]

Hi, I thought loading workflows from YAML is already supported, see this sample: https://github.com/google/adk-python/tree/main/contributing/samples/workflows/loop_config

What new features does this PR bring?

[baptmont]

Hi @boyangsvl, trying to run the example you linked does not work, at least using adk run.
I am getting
ValueError: Fail to load '/contributing/samples/workflows/loop_config/root_agent.yaml' config. Invalid fully qualified name: google.adk.agents.Workflow

Changing the agent_class makes little progress

agent_class: Workflow #original

agent_class: google.adk.workflow.Workflow #change

I get Invalid agent class google.adk.workflow.Workflow. It must be a subclass of BaseAgent.

Even changing the _resolve_agent_class return type is not enough, it just gives new errors.

def _resolve_agent_class(agent_class: str) -> type[BaseAgent]: #original

def _resolve_agent_class(agent_class: str) -> type[BaseNode]: #change

Please let me know if I am missing something, or if the feature is actually missing

Also this PR does not use the logic defined in the example README.md, it expects a yaml file for tools, functions and joins.

agent_class: ToolNode
or
agent_class: JoinNode
or
agent_class: FunctionNode

If the feature is missing I can update this PR to follow the README.md logic of
"Fields ending with _code (like output_schema_code in evaluate_headline.yaml) tell the ADK YAML mapper to resolve the value as a Python code reference rather than treating it as a plain string."
"If a string in the edge list does not end with .yaml and is not 'START', it is treated as a function reference.".