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/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},