test: update tests for new session_id behavior

session_id "new" now returns null at creation (set from SDK after completion).
Session tests use custom session_ids directly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_api_requests.py

@@ -49,8 +49,8 @@ async def test_create_request_with_new_session(client, topic_id):
     )
     assert resp.status_code == 202
     data = resp.json()
-    assert data["data"]["session_id"] is not None
-    assert data["data"]["session_id"] != "new"
+    # session_id is None at creation; gets set from SDK after completion
+    assert data["data"]["session_id"] is None
 
 
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