fix: ToC so it doesn’t include itself or duplicate the end marker (#4388)

turns out the ToC was including itself when generating, which messed up
comparisons and sometimes made the file rewrite endlessly.

also fixed the slice so `<!-- End ToC -->` doesn’t get duplicated when
we insert the new ToC.

should behave nicely now - no extra rewrites, no doubled markers.

Co-authored-by: Eric Traut <etraut@openai.com>

scripts/readme_toc.py

@@ -87,8 +87,9 @@ def check_or_fix(readme_path: Path, fix: bool) -> int:
     # extract current ToC list items
     current_block = lines[begin_idx + 1 : end_idx]
     current = [l for l in current_block if l.lstrip().startswith("- [")]
-    # generate expected ToC
-    expected = generate_toc_lines(content)
+    # generate expected ToC from content without current ToC
+    toc_content = lines[:begin_idx] + lines[end_idx+1:]
+    expected = generate_toc_lines("\n".join(toc_content))
     if current == expected:
         return 0
     if not fix:
@@ -108,7 +109,7 @@ def check_or_fix(readme_path: Path, fix: bool) -> int:
         return 1
     # rebuild file with updated ToC
     prefix = lines[: begin_idx + 1]
-    suffix = lines[end_idx:]
+    suffix = lines[end_idx+1:]
     new_lines = prefix + [""] + expected + [""] + suffix
     readme_path.write_text("\n".join(new_lines) + "\n", encoding="utf-8")
     print(f"Updated ToC in {readme_path}.")