feat(disputed): Confirm My Validation button + discussion request modal

Backend: POST /tests/{id}/request-discussion
  - Only callable by the lead whose vote is 'approved' in a disputed test
  - Sends notification to the rejecting lead: 'Lead X confirms their
    approval and wants to discuss your rejection'
  - Logs the action in audit trail

Frontend:
- 'Confirm My Validation' button (amber outline) alongside 'Change to Rejected'
- Opens a modal showing:
    * Explanation: both leads must agree to finalise
    * Other lead's rejection reason/notes
    * What happens next (stays disputed, notification sent, either can change)
- 'Send Discussion Request' → calls the new endpoint → shows success state:
    'Lead username has been notified...'
- Instruction to reach out via team channels to resolve offline

Flow summary for disputed tests:
  Approving lead sees 2 options:
    a) 'Confirm My Validation' → modal → send request → other lead notified
    b) 'Change to Rejected' → validation modal → both agree to reject → rejected

frontend/src/api/tests.ts

@@ -308,6 +308,16 @@ export interface RTImportResult {
   engagement: string;
 }
 
+/** Confirm approval in a disputed test and notify the rejecting lead to discuss. */
+export async function requestDiscussion(testId: string): Promise<{
+  status: string;
+  message: string;
+  rejector_username: string;
+}> {
+  const { data } = await client.post(`/tests/${testId}/request-discussion`);
+  return data;
+}
+
 /** Import results from a real Red Team engagement. */
 export async function importRT(payload: RTImportPayload): Promise<RTImportResult> {
   const { data } = await client.post<RTImportResult>("/tests/import-rt", payload);