feat: add comprehensive unit tests for FragmentClient methods; include shared test constants

tests/conftest.py

@@ -3,6 +3,9 @@ import os
 
 import pytest
 
+from pyfragment import FragmentClient
+from tests.shared import VALID_API_KEY, VALID_COOKIES, VALID_SEED
+
 
 @pytest.fixture
 def cookies():
@@ -14,3 +17,9 @@ def cookies():
         return json.loads(raw)
     except Exception as exc:
         pytest.skip(f"Cookies unavailable — {exc}")
+
+
+@pytest.fixture
+def client() -> FragmentClient:
+    """Pre-built FragmentClient with dummy credentials."""
+    return FragmentClient(seed=VALID_SEED, api_key=VALID_API_KEY, cookies=VALID_COOKIES)