docs(notion-writer): fix Task 2 upload tests to mock builtins.open

upload() opens the file before subprocess.run; the two upload tests must
mock open() so the nonexistent /tmp paths don't raise FileNotFoundError.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/superpowers/plans/2026-06-27-notion-writer-cli-enhancements.md

@@ -165,7 +165,10 @@ def test_upload_returns_id():
         stdout="43833259-72ae-404e-8441-b6577f3159b4\tphoto.png\tuploaded\n",
         stderr="",
     )
-    with mock.patch("subprocess.run", return_value=fake):
+    # upload() opens the file before subprocess.run; mock open so the file
+    # need not exist (subprocess.run is mocked and never reads the handle).
+    with mock.patch("builtins.open", mock.mock_open(read_data=b"x")), \
+         mock.patch("subprocess.run", return_value=fake):
         upload_id = ntn_files.upload(Path("/tmp/photo.png"))
         assert upload_id == "43833259-72ae-404e-8441-b6577f3159b4"
 
@@ -175,7 +178,8 @@ def test_upload_failure_raises():
     fake = subprocess.CompletedProcess(
         args=[], returncode=1, stdout="", stderr="boom: invalid file",
     )
-    with mock.patch("subprocess.run", return_value=fake):
+    with mock.patch("builtins.open", mock.mock_open(read_data=b"x")), \
+         mock.patch("subprocess.run", return_value=fake):
         try:
             ntn_files.upload(Path("/tmp/bad.png"))
             assert False, "expected NtnUploadError"