fix(plugin): restore menu__list-item--deprecated class on sidebar items

The MDX front matter template emitted `sidebar_class_name` without the
deprecated modifier. Under Docusaurus 3.10+ (bundled with v5.0.0), front
matter takes precedence over sidebar item className, clobbering the
deprecated class set by the sidebar generator. Conditionally append the
class in the template so it survives the precedence change.

Closes #1417

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: sserrata

demo/templates/api.mustache

@@ -19,7 +19,7 @@ hide_table_of_contents: true
 api: {{{json}}}
 {{/json}}
 {{#api.method}}
-sidebar_class_name: "{{{api.method}}} api-method"
+sidebar_class_name: "{{{api.method}}} api-method{{#api.deprecated}} menu__list-item--deprecated{{/api.deprecated}}"
 {{/api.method}}
 {{#infoPath}}
 info_path: {{{infoPath}}}

packages/docusaurus-plugin-openapi-docs/src/index.ts

@@ -223,7 +223,7 @@ hide_table_of_contents: true
 api: {{{json}}}
 {{/json}}
 {{#api.method}}
-sidebar_class_name: "{{{api.method}}} api-method"
+sidebar_class_name: "{{{api.method}}} api-method{{#api.deprecated}} menu__list-item--deprecated{{/api.deprecated}}"
 {{/api.method}}
 {{#infoPath}}
 info_path: {{{infoPath}}}