refactor: move llm tests to separate integration and mocked files and centralize fixtures in conftest.py

test/test_llm_session_integration.py

@@ -0,0 +1,10 @@
+import pytest
+from photools3.llm_session import LlmSession
+from photools3.photo import Photo
+
+@pytest.mark.skipif(not LlmSession(system_prompt="").is_api_reachable(), reason="API not available")
+def test_llm_session(llm_session, test_photo):
+    result = llm_session.call(
+        prompt="keywords for the attached picture", photos=[test_photo]
+    )
+    assert result