feat(context): add on_workflow_yield hook

[xujustinj]

Summary

• Adds on_workflow_yield to Context, called when a workflow raises WorkflowYield (i.e. one or more nodes yielded and execution cannot complete)
• Both TopologicalExecutionAlgorithm and ParallelExecutionAlgorithm call the hook before re-raising WorkflowYield
• Adds TestOnWorkflowYield to tests/test_hooks.py with 4 cases × 2 algorithms (8 new tests)

Test plan

• Hook is called when the workflow yields
• Hook receives the correct workflow, input, and exception arguments (with exception.node_yields populated)
• Hook is not called on successful completion
• Hook is not called when the workflow errors

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