Migrate dev dependencies to [dependency-groups]

Allows `uv sync --dev` to work as expected instead of requiring `--extra dev`.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: mdz

pyproject.toml

@@ -12,7 +12,7 @@ dependencies = [
     "python-dateutil>=2.8.1"
 ]
 
-[project.optional-dependencies]
+[dependency-groups]
 dev = [
     "pytest",
     "pytest-asyncio",