fix(test-dispatch): fall back to tracker defaults, surface soft errors

- dispatch_test_notification now resolves tracking_config / template_config
  from the tracker's default_* fields when the per-link override is unset,
  matching what load_link_data does for the real watcher. Previously
  periodic/scheduled/memory tests silently failed with "no template
  defined" whenever the user configured the template config at the
  tracker level instead of on each link (the UI's normal default).
- Distinguish the two missing-template cases in the returned error
  ("no template config linked" vs. "slot missing in linked config").
- Frontend testTrackerTarget now treats {success:false,error:"..."} in a
  2xx body as a failure — previously any 2xx flashed a success snack so
  users never saw the real reason their test didn't deliver.

frontend/src/routes/notification-trackers/+page.svelte

@@ -358,8 +358,19 @@
 		if (ttTesting[key]) return;
 		ttTesting = { ...ttTesting, [key]: testType };
 		try {
-			await api(`/notification-trackers/${trackerId}/targets/${ttId}/test/${testType}?locale=${getLocale()}`, { method: 'POST' });
+			// The endpoint returns 200 OK with ``{success: false, error: "..."}``
-			snackSuccess(t('snack.targetTestSent'));
+			// on soft failures (missing template slot, no matching assets,
+			// provider unreachable, etc.), so checking for a thrown exception
+			// is not enough.  Surface ``error`` as a snackError when present.
+			const res = await api<{ success?: boolean; error?: string; target?: string }>(
+				`/notification-trackers/${trackerId}/targets/${ttId}/test/${testType}?locale=${getLocale()}`,
+				{ method: 'POST' },
+			);
+			if (res && res.success === false) {
+				snackError(res.error || t('common.error'));
+			} else {
+				snackSuccess(t('snack.targetTestSent'));
+			}
 		} catch (err: any) {
 			snackError(err.message);
 		} finally {

packages/server/src/notify_bridge_server/services/test_dispatch.py

@@ -55,17 +55,21 @@ async def dispatch_test_notification(
     provider_config = dict(provider.config)
     collection_ids = list(tracker.collection_ids or [])
 
-    # Load tracking config
+    # Resolve tracking config: per-link override, else the tracker's default.
+    # The real watcher applies this fallback in ``load_link_data`` — tests
+    # must use the same logic or the user's per-tracker defaults look broken.
+    tracking_config_id = tt.tracking_config_id or tracker.default_tracking_config_id
     tracking_config = None
-    if tt.tracking_config_id:
+    if tracking_config_id:
-        tracking_config = await session.get(TrackingConfig, tt.tracking_config_id)
+        tracking_config = await session.get(TrackingConfig, tracking_config_id)
 
-    # Load template slots keyed by EventType.SCHEDULED_MESSAGE.value
+    # Same fallback for template config.
+    template_config_id = tt.template_config_id or tracker.default_template_config_id
     template_config = None
     template_slots: dict[str, dict[str, str]] | None = None
     slot_name = _TEST_TYPE_SLOT_MAP.get(test_type, test_type)
-    if tt.template_config_id:
+    if template_config_id:
-        template_config = await session.get(TemplateConfig, tt.template_config_id)
+        template_config = await session.get(TemplateConfig, template_config_id)
         if template_config:
             slot_result = await session.exec(
                 select(TemplateSlot).where(
@@ -97,10 +101,21 @@ async def dispatch_test_notification(
     )
 
     if not template_slots:
+        if not template_config_id:
+            return {
+                "success": False,
+                "error": (
+                    "This tracker has no Template Config linked (neither on the "
+                    "tracker's default nor on this target link). Assign one in the "
+                    "tracker settings and make sure it defines a "
+                    f"'{slot_name}' slot."
+                ),
+            }
         return {
             "success": False,
             "error": (
-                f"No '{slot_name}' template defined for this target's template config "
+                f"No '{slot_name}' template defined in the linked Template Config "
+                f"'{template_config.name if template_config else template_config_id}' "
                 f"(locale: {locale}). Add the slot under Template Configs."
             ),
         }