feat: expose operation attempt in contexts

[zhongkechen]

Summary

• expose the 1-based current attempt on StepContext
• expose the same attempt on WaitForConditionCheckContext
• replace process-global retry counters in the step retry examples with durable attempt-based behavior
• add regression coverage proving retry behavior is independent of warm Lambda process state

Testing

• hatch run test:all (2887 passed, 2 skipped)
• hatch run dev-examples:test (81 passed, 2 skipped)
• affected retry example tests on the final rebased branch (4 passed)
• hatch run types:check
• hatch fmt --check (core and examples packages)

Closes #526
Closes #520