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.

README.md

@@ -0,0 +1,22 @@
+# eBay REST Client
+
+This workspace contains a Python-first eBay REST client foundation with:
+
+- a shared OAuth 2.0 core
+- a shared HTTP transport layer
+- public API wrappers per eBay REST domain
+- an isolated OpenAPI generation script for each contract in this folder
+
+## Generate Low-Level Clients
+
+Run the generator script from the project root:
+
+```powershell
+& .\.venv\Scripts\python.exe .\scripts\generate_clients.py
+```
+
+To generate only one API package:
+
+```powershell
+& .\.venv\Scripts\python.exe .\scripts\generate_clients.py --api notification
+```