fix(extensions): reject duplicate provides.templates/scripts names (#4016)

The resolver returns the first entry matching a declared name, so a
later duplicate within provides.templates or provides.scripts was
silently unreachable while still counted by ExtensionManifest
properties. Reject duplicates at manifest-validation time instead.

Also clarify EXTENSION-DEVELOPMENT-GUIDE.md's provides section: hooks
and events are top-level manifest fields, not provides sub-fields, so
the "at least one of ..." wording doesn't imply they can be nested
under provides.

tests/test_extensions.py

@@ -1162,6 +1162,29 @@ class TestExtensionManifestTemplatesAndScripts:
         with pytest.raises(ValidationError, match="must be lowercase alphanumeric with hyphens only"):
             ExtensionManifest(manifest_path)
 
+    @pytest.mark.parametrize("section", ["templates", "scripts"])
+    def test_provides_entry_duplicate_name_rejected(self, temp_dir, valid_manifest_data, section):
+        """Two entries in the same section sharing a name are rejected.
+
+        The resolver (PresetResolver._extension_manifest_declared_template)
+        returns the first entry matching a name, so a later duplicate would
+        be silently unreachable while still counted by ExtensionManifest
+        properties -- reject it up front instead.
+        """
+        import yaml
+
+        valid_manifest_data["provides"][section] = [
+            {"name": "dup", "file": f"{section}/a.txt"},
+            {"name": "dup", "file": f"{section}/b.txt"},
+        ]
+
+        manifest_path = temp_dir / "extension.yml"
+        with open(manifest_path, 'w', encoding="utf-8") as f:
+            yaml.dump(valid_manifest_data, f)
+
+        with pytest.raises(ValidationError, match=f"Duplicate .* name 'dup' in 'provides.{section}'"):
+            ExtensionManifest(manifest_path)
+
     @pytest.mark.parametrize("section", ["templates", "scripts"])
     def test_provides_entry_path_traversal_rejected(self, temp_dir, valid_manifest_data, section):
         """The 'file' field is checked with the same path-safety policy as commands."""