Fix workspace diff multi-patch rendering

packages/plugins/plugin-workspace-diff/tests/diff-model.spec.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 import {
   buildFilePatch,
+  buildFilePatches,
   diffSummary,
   nextExpandedFileSet,
   statusLabel,
@@ -47,6 +48,64 @@ describe("workspace diff UI model", () => {
     });
   });
 
+  it("keeps staged and unstaged patches renderable as separate single-file diffs", () => {
+    const stagedPatch = [
+      "diff --git a/src/app.ts b/src/app.ts",
+      "index 1111111..2222222 100644",
+      "--- a/src/app.ts",
+      "+++ b/src/app.ts",
+      "@@ -1 +1 @@",
+      "-export const value = 1;",
+      "+export const value = 2;",
+      "",
+    ].join("\n");
+    const unstagedPatch = [
+      "diff --git a/src/app.ts b/src/app.ts",
+      "index 2222222..3333333 100644",
+      "--- a/src/app.ts",
+      "+++ b/src/app.ts",
+      "@@ -3 +3 @@",
+      "-export const label = 'old';",
+      "+export const label = 'new';",
+      "",
+    ].join("\n");
+    const file = changedFile({
+      staged: true,
+      unstaged: true,
+      patches: [
+        {
+          kind: "staged",
+          patch: stagedPatch,
+          additions: 1,
+          deletions: 1,
+          binary: false,
+          oversized: false,
+          truncated: false,
+          warnings: [],
+        },
+        {
+          kind: "unstaged",
+          patch: unstagedPatch,
+          additions: 1,
+          deletions: 1,
+          binary: false,
+          oversized: false,
+          truncated: false,
+          warnings: [],
+        },
+      ],
+    });
+
+    const patches = buildFilePatches(file);
+    const viewModel = toFileViewModels(diffResponse({ files: [file] }))[0];
+
+    expect(buildFilePatch(file)).toBe(stagedPatch.trimEnd());
+    expect(patches.map((patch) => patch.kind)).toEqual(["staged", "unstaged"]);
+    expect(patches.map((patch) => patch.patch?.match(/^diff --git/gm)?.length ?? 0)).toEqual([1, 1]);
+    expect(viewModel?.patches).toHaveLength(2);
+    expect(viewModel?.patchKinds).toEqual(["staged", "unstaged"]);
+  });
+
   it("toggles expanded file state without mutating the current set", () => {
     const current = new Set(["a.ts"]);
     const collapsed = nextExpandedFileSet(current, "a.ts");