cargo.build: write the SDK copy in the cache only when a build needs it

An unwritable cache makes only a build fail, with the folder in
data.output. The host still starts.

cli/tests/cargo.rs

@@ -268,6 +268,42 @@ async fn native_discovery_and_error_paths() {
     );
     host.stop().await;
 }
+/// Rework 3: an unavailable cache prevents builds, not host startup or SDK installation.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn cache_failure_is_deferred_until_build() {
+    let world = TestWorld::new();
+    world.source("simple", &fixture("cargo-test-fail"));
+    // A file blocks directory creation even when the test runs as root.
+    let cache = world.0.join("blocked-cache");
+    std::fs::write(&cache, "not a directory").unwrap();
+    let host = Host::start_with_cache(&world.0, &cache).await.unwrap();
+    assert!(world.0.join("plugins/cargo/sdk/src/lib.rs").is_file());
+    let error = host
+        .call("cargo.build", json!({"plugin":"simple"}))
+        .await
+        .unwrap_err();
+    assert_eq!(error["code"], -32004);
+    let output = error["data"]["output"].as_str().unwrap();
+    assert!(output.contains(cache.to_str().unwrap()), "{output}");
+    assert!(output.contains("create"), "{output}");
+    host.stop().await;
+
+    let standalone = TestWorld::new();
+    standalone.source("simple", &fixture("cargo-test-fail"));
+    let builder = sausage::cargo::Cargo::with_runner(
+        World::open(&standalone.0).unwrap(),
+        PathBuf::from(env!("CARGO_BIN_EXE_sausage")),
+        &cache,
+    )
+    .unwrap();
+    assert!(standalone.0.join("plugins/cargo/sdk/src/lib.rs").is_file());
+    let error = builder
+        .build(&standalone.0.join("plugins/simple/source"))
+        .await
+        .unwrap_err();
+    assert!(error.output.contains(cache.to_str().unwrap()), "{error}");
+    assert!(cache.is_file());
+}
 /// T23: release-only failures use data.output and retain successful WASI totals.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn release_only_failure() {