test: render plugin templates through render_template with the plugin's folder in front (#5122)

Calling app.update_template_context directly failed in CI (run 8386:
"missing 1 required positional argument: 'context'"). Rather than
reproduce Quart's render path, the helper puts the plugin's template
folder ahead of the app's loader for the one render and calls
quart.render_template, which supplies the request context as it does
for every page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/_templates.py

@@ -3,6 +3,7 @@ way to render a plugin's template in a test."""
 import pathlib
 
 from jinja2 import ChoiceLoader, FileSystemLoader
+from quart import render_template
 
 import steward
 
@@ -16,11 +17,14 @@ def template_files() -> list[pathlib.Path]:
 
 
 async def render_plugin_template(app, plugin: str, name: str, **ctx) -> str:
-    """Render plugins/<plugin>/templates/<name> through the app's own Jinja
-    environment (its globals, filters and request context), whether or not the
-    plugin is enabled in this app. Call inside app.test_request_context()."""
-    env = app.jinja_env.overlay(loader=ChoiceLoader(
-        [FileSystemLoader(str(REPO / "plugins" / plugin / "templates")), app.jinja_env.loader]))
-    await app.update_template_context(ctx)
-    template = env.get_template(name)
-    return await template.render_async(**ctx) if env.is_async else template.render(**ctx)
+    """Render plugins/<plugin>/templates/<name> with quart.render_template,
+    whether or not the plugin is enabled in this app: the plugin's template
+    folder goes ahead of the app's loader for the one render. Call inside
+    app.test_request_context()."""
+    loader = app.jinja_env.loader
+    app.jinja_env.loader = ChoiceLoader(
+        [FileSystemLoader(str(REPO / "plugins" / plugin / "templates")), loader])
+    try:
+        return await render_template(name, **ctx)
+    finally:
+        app.jinja_env.loader = loader