精準修復 Surgical Patch

The bug is in how the start index is calculated. start = page * size treats page as 0-based, so page 2 starts at index 4 and returns [4, 5]. For a 1-based page, the start index needs to be (page - 1) * size.

--- a/utils/pagination.py
+++ b/utils/pagination.py
@@ -2,7 +2,7 @@ def paginate(items, page, size):
     """Return the items on a 1-based page."""
     if size <= 0:
         raise ValueError("size must be positive")
-    start = page * size
+    start = (page - 1) * size
     end = start + size
     return items[start:end]
 
--- a/tests/test_pagination.py
+++ b/tests/test_pagination.py
@@ -3,2 +3,10 @@ from utils.pagination import paginate, page_count
 def test_page_count():
     assert page_count([1, 2, 3, 4, 5], 2) == 3
+
+
+def test_paginate_is_one_based():
+    items = [1, 2, 3, 4, 5]
+    assert paginate(items, page=1, size=2) == [1, 2]
+    assert paginate(items, page=2, size=2) == [3, 4]
+    assert paginate(items, page=3, size=2) == [5]
+    assert paginate(items, page=4, size=2) == []

The new test checks the case you reported (page 2), plus the first page, the partial last page, and a page past the end. I couldn't run it because the files aren't in this workspace. After you apply the diff, run pytest tests/test_pagination.py. The new test should fail on the old code and pass with the fix.

Something to consider: this diff only fixes the off-by-one. It still doesn't check that page is valid. Now that pages start at 1, page=0 gives start = -size, so it would return a slice taken from the end of the list instead of failing. If you want that rejected, add if page < 1: raise ValueError("page must be >= 1") next to the existing size check. I didn't include it because it changes behavior beyond the bug you reported.