fix(jira,tests): send real datetime to RT/BT date fields, resolve assignee usernames for operators

- RT/BT Start/End Date are genuine Jira "datetime" custom fields (confirmed
  via issue_editmeta), not plain dates. Pushing a bare "YYYY-MM-DD" string
  made Jira default the time-of-day to midnight, so RT Start Date and RT
  End Date ended up showing the same meaningless midnight timestamp instead
  of the actual execution window. Now sends a full ISO datetime.
- The test detail page showed "RT: Unassigned" for the operator who WAS
  correctly assigned, because GET /users/operators (the only source
  AssigneeControl used to resolve an assignee ID into a username) is
  restricted to leads/managers — a plain operator has no other way to
  resolve their own ID. TestOut now resolves and includes the assignee's
  username directly (red/blue tech + reviewer), so the badge no longer
  depends on a permission-gated list the viewer might not have access to.

backend/tests/test_jira_service.py

@@ -397,8 +397,8 @@ def test_push_rt_submitted_uses_execution_times_not_click_time(mock_get_client,
     jira_service.push_rt_submitted(db, test)
 
     fields = mock_jira.update_issue_field.call_args[1]["fields"]
-    assert fields[jira_service.JIRA_FIELD_RT_START_DATE] == "2026-03-01"
-    assert fields[jira_service.JIRA_FIELD_RT_END_DATE] == "2026-03-01"
+    assert fields[jira_service.JIRA_FIELD_RT_START_DATE] == "2026-03-01T09:00:00.000+0000"
+    assert fields[jira_service.JIRA_FIELD_RT_END_DATE] == "2026-03-01T11:00:00.000+0000"
 
 
 @patch("app.services.jira_service.has_admin_jira_configured", return_value=True)
@@ -439,8 +439,8 @@ def test_push_rt_submitted_rt_start_date_survives_reopen(mock_get_client, mock_c
     jira_service.push_rt_submitted(db, test)
 
     fields = mock_jira.update_issue_field.call_args[1]["fields"]
-    assert fields[jira_service.JIRA_FIELD_RT_START_DATE] == "2026-03-01"
-    assert fields[jira_service.JIRA_FIELD_RT_END_DATE] == "2026-03-05"
+    assert fields[jira_service.JIRA_FIELD_RT_START_DATE] == "2026-03-01T09:00:00.000+0000"
+    assert fields[jira_service.JIRA_FIELD_RT_END_DATE] == "2026-03-05T10:00:00.000+0000"
 
 
 @patch("app.services.jira_service.has_admin_jira_configured", return_value=True)