test(server): avoid float equality in usage scenario

Use a tolerance-based runtime assertion so workspace clippy can run with float_cmp denied.

lib/crates/fabro-server/tests/it/scenario/usage.rs

@@ -150,5 +150,8 @@ fn assert_non_llm_billing(billing: &serde_json::Value, expected_stage_ids: &[&st
     let total_runtime_secs = billing["totals"]["runtime_secs"]
         .as_f64()
         .expect("totals should include runtime_secs");
-    assert_eq!(total_runtime_secs, runtime_secs);
+    assert!(
+        (total_runtime_secs - runtime_secs).abs() < f64::EPSILON,
+        "total runtime {total_runtime_secs} should equal summed stage runtime {runtime_secs}"
+    );
 }