build: move pytest/pytest-cov/pytest-mock from runtime deps to dev group (#29)

These three packages were declared in [project].dependencies, which forced every consumer that ran `pip install google-colab-cli` to also install the entire pytest test framework plus its transitive deps (coverage, iniconfig, pluggy) -- ~2.5MB of test machinery that user code never imports. The correct semantic for test dependencies is [dependency-groups].dev (PEP 735), which `uv sync` populates by default for contributors but is NOT advertised in the published wheel's metadata as a runtime requirement. Concrete impact measured by installing the resulting wheel into a fresh py3.13 venv (`uv pip install dist/*.whl`): Before: 57 packages, 200 MB site-packages After: 51 packages, 197 MB site-packages Gone: pytest, pytest-cov, pytest-mock, coverage, iniconfig, pluggy Beyond size, removing pytest from runtime deps stops dependency scanners (pipdeptree, pip-audit, dependabot, snyk) from treating pytest version pins as user-facing constraints -- previously, any downstream environment that pinned pytest 8.x for plugin compat would conflict with our floor of pytest>=9.0.2 even though no user code imports pytest. Verified: - `uv sync` + `uv run pytest tests/` -> 201 passed (dev group still installs everything contributors need) - `uv run ruff check .` -> clean - Built wheel + installed into fresh venv: no pytest/coverage/pluggy in site-packages, `colab version` works, `colab url` works (verifies #27 fragment fix is intact in the install path).