test(shared): add unit tests for BranchDropdown component

frontend/src/components/__tests__/shared/BranchDropdown.spec.js

@@ -0,0 +1,72 @@
+/**
+ * @vitest-environment jsdom
+ * @vitest-environment-options { "url": "https://hub.opencsg.com/models/test-namespace/test-repo" }
+ */
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { mount, flushPromises } from "@vue/test-utils";
+import BranchDropdown from "../../shared/BranchDropdown.vue";
+
+let { mockFetchApi } = vi.hoisted(() => {
+  return {
+    mockFetchApi: vi.fn(() => ({
+      json: () => Promise.resolve({
+        data: { value: { data: [{ name: "main" }, { name: "dev" }] } },
+        error: { value: null }
+      })
+    }))
+  };
+});
+
+vi.mock('../../../packs/useFetchApi', () => ({
+  default: mockFetchApi
+}));
+
+const mockResetFileNotFound = vi.fn();
+vi.mock('../../../stores/RepoTabStore', () => ({
+  useRepoTabStore: () => ({
+    resetFileNotFound: mockResetFileNotFound
+  })
+}));
+
+describe("BranchDropdown", () => {
+  let wrapper;
+
+  beforeEach(async () => {
+    mockFetchApi.mockClear();
+    mockResetFileNotFound.mockClear();
+
+    wrapper = mount(BranchDropdown, {
+      props: {
+        currentBranch: "main"
+      }
+    });
+  });
+
+  it("mounts correctly", () => {
+    expect(wrapper.exists()).toBe(true);
+  });
+
+  it("fetches branches on mount and resolves them", async () => {
+    expect(mockFetchApi).toHaveBeenCalled();
+    await flushPromises();
+    expect(wrapper.vm.branches).toEqual([{ name: "main" }, { name: "dev" }]);
+  });
+
+  it("triggers changeBranch event and resets file not found state when clicked", async () => {
+    await flushPromises();
+    await wrapper.vm.handleClick("dev");
+    expect(wrapper.emitted().changeBranch[0]).toEqual(["dev"]);
+    expect(mockResetFileNotFound).toHaveBeenCalled();
+  });
+
+  it("automatically emits changeBranch to default branch if props.currentBranch does not exist in resolved branch list", async () => {
+    // Reset wrapper with non-existent current branch
+    wrapper = mount(BranchDropdown, {
+      props: {
+        currentBranch: "non-existent"
+      }
+    });
+    await flushPromises();
+    expect(wrapper.emitted().changeBranch[0]).toEqual(["main"]); // fallback to index 0 branch
+  });
+});