Motivation and Context

When a parent workflow has checkpointing enabled but a sub-workflow does not, the checkpoint/restore behavior is undefined. The current implementation in WorkflowExecutor.on_checkpoint_restore() manually rehydrates sub-workflow state by accessing the private _runner_context object, with a TODO acknowledging this is a temporary solution. This
PR adds validation to detect the mismatch early and error clearly, rather than letting developers hit undefined behavior at runtime.

Breaking changes

1. Checkpoint configuration validation: Previously, having a parent workflow with checkpointing and a sub-workflow without it was silently accepted but produced undefined checkpoint/restore
   behavior. Now this raises WorkflowValidationError with validation_type="checkpoint_configuration" at build time (or at runtime if checkpoint_storage is provided only via workflow.run()).

2. ValidationTypeEnum removed: WorkflowValidationError.validation_type is now a ValidationType string literal (e.g., "edge_duplication", "checkpoint_configuration") instead of a
   ValidationTypeEnum member. Compare directly against strings instead of enum values.

Migration for checkpoint validation: pass checkpoint_storage to every sub-workflow's WorkflowBuilder when the parent workflow uses checkpointing.

# Before (undefined behavior on restore)
sub_wf = WorkflowBuilder(start_executor="writer").build()
sub_executor = WorkflowExecutor(sub_wf, id="sub")

# After
sub_wf = WorkflowBuilder(start_executor="writer", checkpoint_storage=storage).build()
sub_executor = WorkflowExecutor(sub_wf, id="sub")

Migration for validation type: compare against string literals instead of enum members.

# Before
from agent_framework import ValidationTypeEnum

except WorkflowValidationError as e:
    if e.validation_type == ValidationTypeEnum.EDGE_DUPLICATION:
        ...

# After
except WorkflowValidationError as e:
    if e.validation_type == "edge_duplication":
        ...

Description

• Adds build-time validation in WorkflowBuilder.build() that checks all WorkflowExecutor children have checkpointing when the parent does
• Adds runtime validation in Workflow._run_core() for the case where checkpoint_storage is provided only at run() time
• Replaces ValidationTypeEnum with ValidationType string literal type for simpler error handling without enum imports
• Updates sub_workflow_checkpoint sample to demonstrate correct explicit configuration
• Closes Python: Handle checkpointing mismatch between parent and sub-workflows #3527

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.