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)

tests/plugins/host_agent/test_host_health.py

@@ -5,10 +5,10 @@ from datetime import datetime, timedelta, timezone
 from types import SimpleNamespace
 
 import pytest
-from quart import render_template
 
 from steward.core.formatting import worst_reading
 from steward.core.settings import to_thresholds_cfg
+from tests._templates import render_plugin_template
 from tests.test_style_tokens import hex_colours
 
 TH = to_thresholds_cfg({})  # cpu/mem/disk warn 80 crit 90; load warn 80 crit 100
@@ -29,8 +29,8 @@ async def _vitals(app, *, cpu=40.0, mem=50.0, stale=False, seen_min=1):
     readings = (("CPU", cpu, "cpu"), ("Memory", mem, "mem"), ("Disk /", 30.0, "disk"),
                 ("Load per core", 20, "load"))
     async with app.test_request_context("/plugins/host_agent/vitals/h1"):
-        page = await render_template(
-            "_host_vitals.html", reporting=True, stale=stale,
+        page = await render_plugin_template(
+            app, "host_agent", "_host_vitals.html", reporting=True, stale=stale,
             reg=SimpleNamespace(last_seen_at=datetime.now(timezone.utc) - timedelta(minutes=seen_min),
                                 agent_version="1.9.0"),
             cpu=cpu, mem=mem, disk_root=30.0, load_per_core=20,
@@ -63,8 +63,8 @@ async def test_a_stale_host_says_since_when_before_any_reading(app):
 async def test_the_metrics_page_grades_by_the_same_cutoffs(app):
     # 75 % was amber under the old hard-coded 70/90; Settings → Thresholds says warn at 80.
     async with app.test_request_context("/plugins/host_agent/h1/metrics"):
-        page = await render_template(
-            "_host_metrics.html", host=SimpleNamespace(address="10.0.0.2"), stale=False,
+        page = await render_plugin_template(
+            app, "host_agent", "_host_metrics.html", host=SimpleNamespace(address="10.0.0.2"), stale=False,
             reg=SimpleNamespace(distro=None, kernel=None, arch=None, agent_version=None,
                                 last_seen_at=datetime.now(timezone.utc)),
             hostlvl={"cpu_pct": 75.0, "mem_used_pct": 95.0, "temp_c_max": 72.0},