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.

tests/test_notification_webhook.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from ebay_client.notification.webhook import WebhookChallengeHandler, WebhookSignatureParser
+
+
+def test_challenge_handler_builds_sha256_response() -> None:
+    response = WebhookChallengeHandler.build_challenge_response(
+        challenge_code="challenge",
+        verification_token="verification",
+        endpoint="https://example.test/webhook",
+    )
+
+    assert len(response) == 64
+
+
+def test_signature_parser_extracts_known_fields() -> None:
+    parser = WebhookSignatureParser()
+
+    parsed = parser.parse("kid=public-key-1,alg=ECDSA,digest=SHA256,sig=Zm9v")
+
+    assert parsed.key_id == "public-key-1"
+    assert parsed.algorithm == "ECDSA"
+    assert parsed.digest == "SHA256"
+    assert parsed.signature == b"foo"