Add tests for OAuth client and webhook notification handling

- Implement tests for `EbayOAuthClient` to verify authorization URL generation with configured scopes and token reuse logic.
- Add tests for `WebhookChallengeHandler` to ensure correct SHA256 response generation and for `WebhookSignatureParser` to validate extraction of known fields from signature strings.

pyproject.toml

@@ -0,0 +1,30 @@
+[build-system]
+requires = ["setuptools>=69", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "ebay-rest-client"
+version = "0.1.0"
+description = "REST-first eBay client with shared OAuth core and isolated OpenAPI-generated packages."
+readme = "README.md"
+requires-python = ">=3.11"
+dependencies = [
+    "cryptography>=44,<46",
+    "httpx>=0.27,<1",
+    "pydantic>=2.8,<3",
+    "PyYAML>=6,<7",
+]
+
+[project.optional-dependencies]
+dev = [
+    "openapi-python-client>=0.25,<0.26",
+    "pytest>=8,<9",
+    "pytest-httpx>=0.35,<0.36",
+]
+
+[tool.setuptools.packages.find]
+include = ["ebay_client*"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["."]