chore: add pytest config, remove Makefile

- Add [tool.pytest.ini_options] to pyproject.toml for standalone testing
- Remove Makefile (replaced by uv run pytest)

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Co-Authored-By: Amplifier <240397093+microsoft-amplifier@users.noreply.github.com>

Author: bkrabach

Makefile

@@ -40,3 +40,9 @@ dev = [
 
 [tool.uv.sources]
 amplifier-core = { git = "https://github.com/microsoft/amplifier-core", branch = "main" }
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+addopts = "--import-mode=importlib"
+asyncio_mode = "strict"
+