chore: move test dependencies from extra to dependency group

[g4bri3lDev]

Summary

pytest, pytest-asyncio, pytest-cov, pytest-xdist, and hypothesis lived in the [project.optional-dependencies] test extra. Extras are not installed by a plain uv run, so uv run pytest in a fresh checkout or git worktree silently fell back to whatever pytest is on $PATH — running the tests under a different interpreter against whatever opendisplay that interpreter could import (in the worst case, a stale editable install of a different checkout), instead of the tree under test.

This moves them to a test dependency group, included in dev, so they are synced by default and uv run pytest always uses the project venv. Test extras are development tooling, not publishable package metadata, so a group is also the semantically right home (uv docs).

No workflow or docs changes needed: test.yml/lint.yml/README all use uv sync --all-extras (± --group dev), which installs the default dev group; nothing referenced the test extra by name.

Test plan

• fresh venv, plain uv run pytest: resolves .venv/bin/pytest and imports opendisplay from the current tree (previously: system pytest + foreign editable install)
• uv run --all-extras pytest -q → 461 passed
• uv lock resolves cleanly; prek hooks (ruff/mypy/pylint) pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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