fix(ci): stabilize graphviz Rust checks

Keep the render-graph CLI integration test explicitly documented for
synchronous stdio subprocess usage, and make the garbage-stdout server
test drain stdin before returning invalid output so the Linux test path
stays deterministic.

lib/crates/fabro-cli/tests/it/cmd/render_graph.rs

@@ -1,3 +1,8 @@
+#![expect(
+    clippy::disallowed_methods,
+    reason = "These CLI integration tests intentionally spawn the real fabro binary and stream DOT over stdio to verify the internal render subprocess contract."
+)]
+
 use std::io::Write;
 use std::process::{Command, Stdio};
 

lib/crates/fabro-server/src/server.rs

@@ -8048,7 +8048,8 @@ slug = "fabro"
     #[cfg(unix)]
     #[tokio::test]
     async fn render_dot_subprocess_returns_protocol_violation_for_garbage_stdout() {
-        let (_dir, script_path) = write_test_executable("#!/bin/sh\nprintf 'garbage'\nexit 0\n");
+        let (_dir, script_path) =
+            write_test_executable("#!/bin/sh\ncat >/dev/null\nprintf 'garbage'\nexit 0\n");
 
         let result = render_dot_subprocess("digraph { a -> b }", Some(&script_path)).await;