fix: make enterprise DashboardContextEnricher fail-safe and add context defaults

After PR #38094 removed enterprise context variables from student_dashboard()
in favor of a DashboardContextEnricher filter pipeline, two issues remained:

1. fail_silently: False could crash the entire dashboard if the enricher is
   unavailable or fails
2. enterprise_message and enterprise_learner_portal_enabled_message had no
   explicit defaults in the context dict

Changes:
- lms/envs/common.py: Changed fail_silently: False to fail_silently: True
  for DashboardContextEnricher to prevent dashboard crashes
- dashboard.py: Added explicit empty-string defaults for enterprise context
  variables so the template contract is always satisfied

Agent-Logs-Url: https://github.com/openedx/openedx-platform/sessions/9dfd421a-df10-4df0-a99f-70d73ea5378e

Co-authored-by: kiram15 <31229189+kiram15@users.noreply.github.com>

Author: Copilot

common/djangoapps/student/views/dashboard.py

@@ -790,6 +790,8 @@ def student_dashboard(request):  # pylint: disable=too-many-statements
     context = {
         'urls': urls,
         'programs_data': programs_data,
+        'enterprise_message': '',
+        'enterprise_learner_portal_enabled_message': '',
         'enrollment_message': enrollment_message,
         'redirect_message': Text(redirect_message),
         'account_activation_messages': account_activation_messages,

lms/envs/common.py

@@ -3292,7 +3292,7 @@ def _should_send_certificate_events(settings):
         "pipeline": ["enterprise.filters.accounts.AccountSettingsReadOnlyFieldsStep"],
     },
     "org.openedx.learning.dashboard.render.started.v1": {
-        "fail_silently": False,
+        "fail_silently": True,
         "pipeline": ["enterprise.filters.dashboard.DashboardContextEnricher"],
     },
 }