Add integration tests for extra_cargo_args

Test that extra cargo args are forwarded to cargo during the prepare
phase: a positive test with --quiet and a negative test with an invalid
flag to prove args are actually passed through.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: enthropy7

tests/buildtest/mod.rs

@@ -247,6 +247,37 @@ fn test_cargo_workspace() {
     });
 }
 
+#[test]
+fn test_extra_cargo_args() {
+    runner::run("hello-world", |run| {
+        run.build(SandboxBuilder::new().enable_networking(false), |builder| {
+            builder
+                .extra_cargo_args(vec!["--quiet".into()])
+                .run(|build| {
+                    build.cargo().args(&["run"]).run()?;
+                    Ok(())
+                })
+        })?;
+        Ok(())
+    });
+}
+
+#[test]
+fn test_extra_cargo_args_invalid() {
+    runner::run("hello-world", |run| {
+        let res = run.build(SandboxBuilder::new().enable_networking(false), |builder| {
+            builder
+                .extra_cargo_args(vec!["--invalid-flag-that-does-not-exist".into()])
+                .run(|_build| Ok(()))
+        });
+        assert!(
+            res.is_err(),
+            "expected extra cargo args to cause a prepare failure"
+        );
+        Ok(())
+    });
+}
+
 test_prepare_error!(
     test_missing_cargotoml,
     "missing-cargotoml",