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.

.gitignore

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+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
+
+# Virtual environments
+.venv/
+venv/
+env/
+ENV/
+
+# Distribution / packaging
+build/
+dist/
+*.egg-info/
+.eggs/
+pip-wheel-metadata/
+
+# Test and coverage output
+.pytest_cache/
+.coverage
+.coverage.*
+htmlcov/
+
+# Type checker / linter caches
+.mypy_cache/
+.ruff_cache/
+.pyre/
+
+# Tooling caches
+.tox/
+.nox/
+
+# IDE / editor files
+.vscode/
+.idea/
+
+# OS files
+.DS_Store
+Thumbs.db