test: group tests into feature-area subdirs

[pratyush618]

Summary

Stacked on top of #149. Once #149 lands, the base will be retargeted to master and this PR's diff will read as a pure file-move.

Groups the 52 flat test_*.py files into 7 feature-area subdirs that match the taxonomy already documented in .claude/skills/testing/SKILL.md:

Subdir	Theme	Files
tests/core/	fundamental queue mechanics	18
tests/worker/	pools, prefork, native async (+ prefork_apps/)	4
tests/resources/	resource DI, interception, proxies, streaming	5
tests/workflows/	workflow DAG tests	9
tests/observability/	events, hooks, webhooks, customization	7
tests/integrations/	Flask/FastAPI/CLI/KEDA	4
tests/dashboard/	dashboard backend endpoints	3

Top-level tests/conftest.py stays — fixtures (queue, run_worker, workflow_worker, poll_until) propagate down via pytest's hierarchical discovery. No __init__.py files at any level.

tests/prefork_apps/ moves to tests/worker/prefork_apps/ so it stays adjacent to test_prefork.py — the test uses Path(__file__).parent to make it importable, which keeps working unchanged.

No CI/config edits needed: testpaths = ["tests"] already covers subdirs, pytest discovers test_*.py recursively, and the path-filter glob tests/** matches.

Test plan

• uv run python -m pytest tests/ --collect-only — 514 tests collected (same count as before).
• uv run python -m pytest tests/workflows/ tests/worker/test_prefork.py — 83 passed, 1 skipped (exercises the prefork_apps import path after the move).
• uv run mypy py_src/taskito/ tests/ --no-incremental — clean.
• uv run ruff check py_src/ tests/ — clean.
• CI runs the full suite.