test: move tests/python -> tests

[pratyush618]

Summary

Drops the redundant python/ segment from the test tree. There's no tests/rust/ (Rust tests live inline beside their crates), so tests/python/ was just dead nesting.

• Renames every file from tests/python/ to tests/ (git rename detection preserves history).
• Drops the empty tests/python/__init__.py (0 bytes, no semantic role; without it pytest's module names become test_basic instead of python.test_basic).
• Keeps tests/prefork_apps/ adjacent to tests/test_prefork.py, which uses Path(__file__).parent to make the package importable.
• Updates path references in pyproject.toml (testpaths), .pre-commit-config.yaml (mypy), CONTRIBUTING.md (pytest), and .github/workflows/ci.yml (paths-filter glob, mypy, pytest, comment).

Pure rename + config update. No code logic changes. A follow-up PR will group the 52 flat test files into feature-area subdirs.

Test plan

• uv run python -m pytest tests/ --collect-only — 514 tests collected.
• uv run python -m pytest tests/test_basic.py tests/test_prefork.py tests/test_workflows_linear.py — 32 passed, 1 skipped.
• uv run mypy py_src/taskito/ tests/ --no-incremental — clean.
• uv run ruff check py_src/ tests/ — clean.
• CI runs the full suite.