fix(workflow): skip tasks blocked by failed dependencies

[saime428]

Summary

• Detect workflow execution stalls when no tasks are running and no pending task can become ready.
• Mark pending tasks blocked by incomplete dependencies as skipped so the workflow can exit cleanly.
• Add a regression test for a failed dependency blocking a downstream report task.

Why

• Fixes a deadlock where a failed task leaves dependent tasks pending forever and start_workflow() never returns.
• Closes [BUG] Workflow deadlocks indefinitely when task fails and blocks dependent tasks #325.

Validation

• python -m compileall src/strands_tools/workflow.py
• pytest tests/test_workflow.py -q
• ruff format --check src/strands_tools/workflow.py tests/test_workflow.py
• ruff check src/strands_tools/workflow.py tests/test_workflow.py
• hatch run test-lint

Notes for reviewers

• This is a narrow fix for the existing workflow tool, not a Graph migration or new workflow feature.

[saime428]

Hi, checking in on this PR. The related issue #325 is still open, and the Secure Integration test is still waiting at check-access-and-checkout. Please let me know if you’d like any changes or additional validation from me.

[saime428]

Following up on this PR. The authorization check passes, while check-access-and-checkout is still waiting on maintainer approval. Please let me know if any changes are needed from my side.