test(hosts): render the host_agent fragments whether or not the plugin is enabled (#5122)

The unit app does not register plugin blueprints, so render_template could
not find _host_vitals.html or _host_metrics.html (TemplateNotFound, CI run
8384). tests/_templates.render_plugin_template renders a plugin's template
through the app's own Jinja environment and request context, with the
plugin's template folder ahead of the app's loader.

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

tests/_templates.py

@@ -1,6 +1,9 @@
-"""Where Steward's templates live, for the guards that read every one."""
+"""Where Steward's templates live, for the guards that read every one, and a
+way to render a plugin's template in a test."""
 import pathlib
 
+from jinja2 import ChoiceLoader, FileSystemLoader
+
 import steward
 
 REPO = pathlib.Path(steward.__file__).parent.parent
@@ -10,3 +13,14 @@ def template_files() -> list[pathlib.Path]:
     """Every first-party .html template: core and each plugin's."""
     roots = [REPO / "steward" / "templates", *(REPO / "plugins").glob("*/templates")]
     return sorted(p for root in roots for p in root.rglob("*.html"))
+
+
+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)