test(workflows): replace polling sleeps with poll_until

[pratyush618]

Summary

Closes the workflow-test gap left open by #146. PR #146 explicitly skipped tests/workflows/ to avoid conflicting with #145's in-flight conftest changes; with both merged, the deferred 4 pollable sleeps can now use the poll_until fixture.

File	Sleep	Replacement
test_workflows_gates.py:30	time.sleep(3) # Let "a" complete	poll_until on approve == WAITING_APPROVAL
test_workflows_gates.py:56	time.sleep(2) # Let "a" complete and gate enter WAITING_APPROVAL	same
test_workflows_gates.py:80	time.sleep(2)	same
test_workflows_fan_out.py:241	time.sleep(2) # Wait for source to complete and fan-out to expand	poll_until on fetch == COMPLETED
test_workflows_subworkflow.py:143	time.sleep(2) # Let sub-workflow submit	poll_until on sub == RUNNING

The two remaining time.sleep calls in slow_task/slow_process task bodies stay — they're intentional pacing (the test asserts the worker handles a slow task) — but they now have one-line comments explaining why so the next audit doesn't re-flag them.

This finishes audit recommendation 6.

Test plan

• uv run python -m pytest tests/workflows/test_workflows_gates.py tests/workflows/test_workflows_fan_out.py tests/workflows/test_workflows_subworkflow.py — 22 passed.
• uv run mypy py_src/taskito/ tests/ --no-incremental — clean.
• uv run ruff check tests/workflows/ — clean.
• CI runs the full suite.