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

backend/app/routers/tests.py

@@ -745,6 +745,97 @@ def sync_tempo(
     return {"results": results}
 
 
+# ---------------------------------------------------------------------------
+# POST /tests/{id}/request-discussion  —  disputed: confirm vote + notify other lead
+# ---------------------------------------------------------------------------
+
+
+@router.post("/{test_id}/request-discussion")
+def request_discussion(
+    test_id: uuid.UUID,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(require_any_role("red_lead", "blue_lead")),
+):
+    """Called when the approving lead confirms their vote in a disputed test.
+
+    Sends a notification to the other lead (who rejected) asking them to
+    discuss and resolve the conflict.  The test remains in 'disputed' state.
+    """
+    from app.models.enums import TestState as ModelTestState
+    from app.models.user import User as UserModel
+    from app.services.notification_service import create_notification
+
+    test = crud_get_test_or_raise(db, test_id)
+
+    if test.state.value != "disputed":
+        from app.domain.errors import BusinessRuleViolation
+        raise BusinessRuleViolation("Test is not in disputed state")
+
+    role = current_user.role
+
+    # Identify who the "other lead" is (the one who rejected)
+    if role == "red_lead" and test.red_validation_status == "approved":
+        # Red approved, Blue rejected → notify Blue Lead who rejected
+        rejector_id = test.blue_validated_by
+        rejector_label = "Blue Lead"
+        requester_label = "Red Lead"
+    elif role == "blue_lead" and test.blue_validation_status == "approved":
+        # Blue approved, Red rejected → notify Red Lead who rejected
+        rejector_id = test.red_validated_by
+        rejector_label = "Red Lead"
+        requester_label = "Blue Lead"
+    else:
+        from app.domain.errors import BusinessRuleViolation
+        raise BusinessRuleViolation(
+            "You are not the approving lead in this conflict or the state is inconsistent"
+        )
+
+    # Look up the rejecting lead's username for the message
+    rejector = (
+        db.query(UserModel).filter(UserModel.id == rejector_id).first()
+        if rejector_id else None
+    )
+    rejector_name = rejector.username if rejector else rejector_label
+
+    # Notify the rejecting lead
+    if rejector_id:
+        try:
+            create_notification(
+                db,
+                user_id=rejector_id,
+                type="validation_conflict",
+                title="Discussion requested on disputed test",
+                message=(
+                    f"{requester_label} ({current_user.username}) is confirming their approval "
+                    f"of test '{test.name}' and wants to discuss your rejection with you. "
+                    f"Please reach out to resolve the disagreement."
+                ),
+                entity_type="test",
+                entity_id=str(test.id),
+            )
+        except Exception as e:
+            import logging
+            logging.getLogger(__name__).warning(
+                "Failed to send discussion notification: %s", e
+            )
+
+    log_action(
+        db,
+        user_id=current_user.id,
+        action="request_dispute_discussion",
+        entity_type="test",
+        entity_id=test.id,
+        details={"test_name": test.name, "rejector": rejector_name},
+    )
+    db.commit()
+
+    return {
+        "status": "notification_sent",
+        "message": f"Discussion request sent to {rejector_name}",
+        "rejector_username": rejector_name,
+    }
+
+
 # ---------------------------------------------------------------------------
 # POST /tests/import-rt  —  bulk import from a real Red Team engagement
 # ---------------------------------------------------------------------------