chore(release): raise error if no smaller version in changelog

Updates _find_insertion_point to raise a ValueError if it cannot find an
existing version in CHANGELOG.md that is smaller than the new version being
inserted. This replaces the previous behavior of appending to the end of the
file, which should not occur in practice.
Also adds a test case to verify this error behavior.

Author: rickeylev

tests/tools/private/release/changelog_news_test.py

@@ -485,6 +485,40 @@ def test_update_changelog_insertion_point(self):
         self.assertTrue(idx_2_2_0 < idx_2_1_0 < idx_2_0_0)
         self.assertIn("Fix in 2.1.0", new_content)
 
+    def test_update_changelog_insertion_point_too_small(self):
+        # Arrange
+        changelog = """# Changelog
+
+{#unreleased}
+## Unreleased
+
+[unreleased]: https://github.com/bazel-contrib/rules_python/releases/tag/unreleased
+
+{#v2-0-0}
+## [2.0.0] - 2026-04-09
+
+[2.0.0]: https://github.com/bazel-contrib/rules_python/releases/tag/2.0.0
+"""
+        changelog_path = self.tmpdir / "CHANGELOG.md"
+        changelog_path.write_text(changelog)
+
+        news_dir = self.tmpdir / "news"
+        news_dir.mkdir()
+        (news_dir / "123.fixed.md").write_text("Fix in 1.0.0")
+
+        # Act & Assert
+        with self.assertRaises(ValueError) as ctx:
+            changelog_news.update_changelog(
+                "1.0.0",
+                "2026-01-01",
+                changelog_path=changelog_path,
+                news_dir=news_dir,
+            )
+        self.assertIn(
+            "Could not find a version in CHANGELOG.md smaller than 1.0.0",
+            str(ctx.exception),
+        )
+
 
 if __name__ == "__main__":
     unittest.main()

tools/private/release/changelog_news.py

@@ -123,7 +123,9 @@ def _find_insertion_point(changelog_content, new_version):
         if parsed_ver < parsed_new_ver:
             return m.start()
 
-    return len(changelog_content)
+    raise ValueError(
+        f"Could not find a version in CHANGELOG.md smaller than {new_version} to insert before."
+    )
 
 
 def _add_news_to_changelog(input_path, output_path, version, entries, release_date):
@@ -275,15 +277,12 @@ def _add_news_to_changelog(input_path, output_path, version, entries, release_da
         # Find insertion point
         insertion_point = _find_insertion_point(changelog_content, version)
 
-        if insertion_point == len(changelog_content):
-            new_content = changelog_content.rstrip() + "\n\n" + new_release_block + "\n"
-        else:
-            new_content = (
-                changelog_content[:insertion_point]
-                + new_release_block
-                + "\n\n"
-                + changelog_content[insertion_point:]
-            )
+        new_content = (
+            changelog_content[:insertion_point]
+            + new_release_block
+            + "\n\n"
+            + changelog_content[insertion_point:]
+        )
         output_path.write_text(new_content, encoding="utf-8")