build: declare pytest as a dev dependency

pyproject.toml had no dev/test dependency group at all, so pytest wasn't
declared anywhere — installing strictly per pyproject.toml (poetry install,
or pip install . in a clean env) left `pytest tests/` failing with
ModuleNotFoundError. Added [tool.poetry.group.dev.dependencies].

pytest-asyncio, though installed locally, isn't actually required: no test
in this suite uses `async def test_...` / @pytest.mark.asyncio, they all
drive async code via plain asyncio.run() inside sync test functions — so
only pytest itself is declared.

Verified: `poetry check` accepts the new section (only pre-existing,
unrelated [tool.poetry] vs [project] deprecation warnings); `python -m build`
still succeeds; the built wheel's METADATA does not list pytest as a runtime
dependency (dev group is correctly excluded from the published package).

Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS

pyproject.toml

@@ -33,6 +33,9 @@ httpx = {extras = ["socks"], version = ">=0.28.1"}
 typing-extensions = ">=4.9.0"
 pydantic = ">=2.5.0,<3.0.0"
 
+[tool.poetry.group.dev.dependencies]
+pytest = ">=7.0"
+
 [tool.poetry.urls]
 Repository = "https://github.com/VectifyAI/PageIndex"
 Homepage = "https://pageindex.ai"