fix(server): a fork's first /compact compacts its inherited history#24172

@@ -4265,7 +4265,8 @@ export const layer: Layer.Layer<ProjectionStoreV2, never, SqlClient.SqlClient> =
               ) AS present
             `;
             return {
-              hasConversation: conversation[0]?.present === 1,
+              // A fork inherits its source's history before it has messages of its own.
+              hasConversation: thread.forkedFrom?.type === "run" || conversation[0]?.present === 1,