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.

frontend/src/components/test-detail/TestDetailHeader.tsx

@@ -559,6 +559,7 @@ export default function TestDetailHeader({
                 side="red"
                 kind="reviewer"
                 assigneeId={test.red_reviewer_assignee}
+                assigneeUsername={test.red_reviewer_assignee_username}
                 operators={operators}
                 canEdit={role === "red_lead" || role === "manager"}
                 isSaving={isAssigningOperator}
@@ -571,6 +572,7 @@ export default function TestDetailHeader({
                 side="blue"
                 kind="reviewer"
                 assigneeId={test.blue_reviewer_assignee}
+                assigneeUsername={test.blue_reviewer_assignee_username}
                 operators={operators}
                 canEdit={role === "blue_lead" || role === "manager"}
                 isSaving={isAssigningOperator}
@@ -581,6 +583,7 @@ export default function TestDetailHeader({
             <AssigneeControl
               side="red"
               assigneeId={test.red_tech_assignee}
+              assigneeUsername={test.red_tech_assignee_username}
               operators={operators}
               canEdit={role === "red_lead" || role === "manager"}
               isSaving={isAssigningOperator}
@@ -590,6 +593,7 @@ export default function TestDetailHeader({
             <AssigneeControl
               side="blue"
               assigneeId={test.blue_tech_assignee}
+              assigneeUsername={test.blue_tech_assignee_username}
               operators={operators}
               canEdit={role === "blue_lead" || role === "manager"}
               isSaving={isAssigningOperator}