feat: Workflow Control Plane support

[saurabhjain1592]

Summary

Adds Workflow Control Plane support to the Python SDK, enabling governance gates at workflow step transitions for external orchestrators.

Features

• Workflow control methods: create_workflow, get_workflow, step_gate, etc.
• LangGraph adapter (AxonFlowLangGraphAdapter) for simplified integration
• WorkflowBlockedError and WorkflowApprovalRequiredError exceptions
• Full async support

New Files

• axonflow/workflow.py - Pydantic models for workflow types
• axonflow/adapters/langgraph.py - LangGraph adapter
• axonflow/adapters/__init__.py - Adapter exports

Usage

from axonflow import AxonFlow
from axonflow.workflow import CreateWorkflowRequest, StepGateRequest, StepType

async with AxonFlow(endpoint="http://localhost:8080") as client:
    # Create workflow
    workflow = await client.create_workflow(
        CreateWorkflowRequest(workflow_name="my-pipeline")
    )
    
    # Check gate
    gate = await client.step_gate(
        workflow.workflow_id, "step-1",
        StepGateRequest(step_name="Generate", step_type=StepType.LLM_CALL)
    )
    
    if gate.is_allowed():
        # Execute step
        await client.mark_step_completed(workflow.workflow_id, "step-1")

Test plan

• Unit tests pass
• Test coverage >80%
• Import works correctly
• LangGraph adapter works

Related to getaxonflow/axonflow-enterprise#834