refactor(chalice): return file_key with first mob (#2980)

ee/api/chalicelib/core/sessions_replay.py

@@ -19,7 +19,23 @@ def __group_metadata(session, project_metadata):
 
 
 def get_pre_replay(project_id, session_id):
+    with pg_client.PostgresClient() as cur:
+        query = cur.mogrify(
+            f"""\
+            SELECT encode(file_key,'hex') AS file_key
+            FROM public.sessions
+            WHERE project_id = %(project_id)s
+                AND session_id = %(session_id)s;""",
+            {"project_id": project_id, "session_id": session_id}
+        )
+        cur.execute(query=query)
+
+        data = cur.fetchone()
+        file_key = None
+        if data is not None:
+            file_key = data['file_key']
     return {
+        'fileKey': file_key,
         'domURL': [sessions_mobs.get_first_url(project_id=project_id, session_id=session_id, check_existence=False)]}