refactor(plugins): stream boundary prep step output

test/scripts/prepare-extension-package-boundary-artifacts.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from "vitest";
+import { createPrefixedOutputWriter } from "../../scripts/prepare-extension-package-boundary-artifacts.mjs";
+
+describe("prepare-extension-package-boundary-artifacts", () => {
+  it("prefixes each completed line and flushes the trailing partial line", () => {
+    let output = "";
+    const writer = createPrefixedOutputWriter("boundary", {
+      write(chunk: string) {
+        output += chunk;
+      },
+    });
+
+    writer.write("first line\nsecond");
+    writer.write(" line\nthird");
+    writer.flush();
+
+    expect(output).toBe("[boundary] first line\n[boundary] second line\n[boundary] third");
+  });
+});