feat(ext/sw): GitHost.writeFileCreateOnly() refuses to overwrite

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

extension/src/service-worker/__tests__/git-host-extensions.test.ts

@@ -59,3 +59,53 @@ describe('lastCommit (GitHub)', () => {
     expect(await host.lastCommit('manifest.enc')).toBeNull();
   });
 });
+
+describe('writeFileCreateOnly (Gitea)', () => {
+  let fetchSpy: ReturnType<typeof vi.spyOn>;
+  beforeEach(() => { fetchSpy = vi.spyOn(globalThis, 'fetch'); });
+
+  it('creates when file does not exist', async () => {
+    fetchSpy
+      .mockResolvedValueOnce(new Response('', { status: 404 }))     // pre-check
+      .mockResolvedValueOnce(new Response('{}', { status: 201 })); // POST
+    const host = new GiteaHost('https://git.example.com', 'user/vault', 'tok');
+    await host.writeFileCreateOnly('manifest.enc', new Uint8Array([1, 2, 3]), 'init');
+    expect(fetchSpy).toHaveBeenCalledTimes(2);
+    expect((fetchSpy.mock.calls[1][1] as RequestInit).method).toBe('POST');
+  });
+
+  it('throws when file already exists', async () => {
+    fetchSpy.mockResolvedValueOnce(new Response(
+      JSON.stringify({ sha: 'abc' }), { status: 200 },
+    ));
+    const host = new GiteaHost('https://git.example.com', 'user/vault', 'tok');
+    await expect(
+      host.writeFileCreateOnly('manifest.enc', new Uint8Array([1]), 'init'),
+    ).rejects.toThrow(/already exists/);
+    expect(fetchSpy).toHaveBeenCalledTimes(1);  // pre-check only, no write
+  });
+});
+
+describe('writeFileCreateOnly (GitHub)', () => {
+  let fetchSpy: ReturnType<typeof vi.spyOn>;
+  beforeEach(() => { fetchSpy = vi.spyOn(globalThis, 'fetch'); });
+
+  it('throws when file already exists', async () => {
+    fetchSpy.mockResolvedValueOnce(new Response(
+      JSON.stringify({ sha: 'abc' }), { status: 200 },
+    ));
+    const host = new GitHubHost('user/vault', 'tok');
+    await expect(
+      host.writeFileCreateOnly('manifest.enc', new Uint8Array([1]), 'init'),
+    ).rejects.toThrow(/already exists/);
+  });
+
+  it('creates when file does not exist', async () => {
+    fetchSpy
+      .mockResolvedValueOnce(new Response('', { status: 404 }))
+      .mockResolvedValueOnce(new Response('{}', { status: 201 }));
+    const host = new GitHubHost('user/vault', 'tok');
+    await host.writeFileCreateOnly('manifest.enc', new Uint8Array([1]), 'init');
+    expect(fetchSpy).toHaveBeenCalledTimes(2);
+  });
+});