fix: Install ruff and mypy explicitly in lint CI job

The lint job was failing with 'ruff: command not found' because
dependency-groups in pyproject.toml require newer pip/setuptools to work.

Install ruff and mypy directly before installing the package to ensure
they're available for the linting steps.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.github/workflows/tests.yml

@@ -21,7 +21,8 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -e ".[dev]"
+        pip install ruff mypy
+        pip install -e .
 
     - name: Run ruff linter
       run: |