Python: Include sub-workflow structure in graph signature for checkpoint validation

[moonbox3]

Motivation and Context

• _compute_graph_signature now recursively includes sub-workflow topology when a WorkflowExecutor is present, so checkpoint validation detects structural changes inside sub-workflows
• Previously, two workflows with identical parent structure but entirely different sub-workflows produced the same signature, making checkpoint resume blind to sub-workflow changes
• Nested sub-workflows are handled automatically since each recursive call inspects its own WorkflowExecutor children

Description

• Closes Python: [Bug]: Subworkflow graph structure not included as part of workflow validation #3782

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.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework/_workflows
_workflow.py	268	26	90%	87, 265–267, 269–270, 288, 292, 320, 422, 597, 618, 644, 671, 673–674, 679–680, 686, 692, 697, 717–719, 732, 809
TOTAL	16607	2059	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3930	225 💤	0 ❌	0 🔥	1m 9s ⏱️

[Copilot]

Pull request overview

Updates workflow checkpoint validation so that the graph signature reflects nested sub-workflow topology when a WorkflowExecutor is used, preventing silent checkpoint resumes after sub-workflow structural changes.

Changes:

• Extend _compute_graph_signature() to recursively incorporate sub-workflow signatures for WorkflowExecutor nodes.
• Add new unit tests to validate checkpoint resume behavior when sub-workflows match vs. change.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
python/packages/core/agent_framework/_workflows/_workflow.py	Includes sub-workflow structure in the graph signature for checkpoint validation.
python/packages/core/tests/workflow/test_checkpoint_validation.py	Adds tests covering checkpoint validation for nested sub-workflows.