fix(testing): deliver due waits on checkpoint

[yaythomas]

Problem

In DurableFunctionTestRunner, a map/parallel branch that waits cannot resume while a sibling branch keeps working. The runner takes an extra invocation that the real service does not take: verified against Lambda in us-west-2, the service resumes the waiting branch inline in a single invocation while the sibling is still running.

Closes #587

Fix

Complete due waits and step retries at checkpoint time, inside CheckpointCore.apply, immediately before the response delta is computed. Each completion touches its operation, so the transition lands in the same checkpoint response and the suspended branch resumes within the running invocation. Between invocations, the existing earliest-pending timer remains the trigger, unchanged.

• Execution.complete_due_operations holds the shared due predicate and transitions (wait STARTED with scheduled_end_timestamp <= now, step PENDING with next_attempt_timestamp <= now)
• CheckpointCore.apply picks up due operations before pinning the paginator, covering both checkpoint entry points identically
• Executor._fire_due_operations delegates to the shared method instead of duplicating the walk
• No new persisted state: the in-memory, filesystem, and sqlite stores behave identically

Tests

• e2e: a wait in one parallel branch completes while the sibling is still working, in a single invocation, on both the skip clock and the wall clock (wait_inline_completion_test.py)
• e2e: a wait completed while suspended is delivered as new on the next invocation, in exactly two invocations, with on_operation_end firing once with is_replayed=False (wait_suspend_replay_test.py, guards the contract previously only covered by the examples plugin/otel tests)
• unit: due-operation walk transitions and touch behavior (execution_wait_retry_test.py)
• unit: checkpoint-time delivery of a due wait in the response delta (processor_test.py)

Verification

• hatch run dev-testing:test 1362 passed
• hatch run dev-testing:typecheck clean
• hatch run dev-core:test 1457 passed
• hatch run dev-examples:pytest --runner-mode=local 85 passed, 2 skipped (pre-existing cloud/X-Ray skips), including test_plugin and test_plugin_on_operation_end_called_for_wait_completed_during_suspend
• hatch fmt --check clean
• lintcommit PASS