[Bug]: Prepared UPDATE on partitioned table with secondary indexes panics when key parameters are strings

### Environment

- Branch: latest `main`
- MO git version: `51656deb6`
- MySQL oracle: `8.0.45`

### Reproduce

```sql
drop database if exists mysql_compat_model14_prepare_partition_idx_min;
create database mysql_compat_model14_prepare_partition_idx_min;
use mysql_compat_model14_prepare_partition_idx_min;

create table orders_p (
tenant int not null,
order_id int not null,
customer_id int not null,
status varchar(12),
amount decimal(10,2),
created_at date,
primary key (tenant, order_id),
key idx_t_status_amount(tenant, status, amount),
key idx_customer(customer_id)
)
partition by hash(tenant) partitions 4;

insert into orders_p values
(2,202,2002,'hold',40.00,'2024-02-02'),
(3,301,3001,'paid',50.00,'2024-03-01');

prepare s_upd from
'update orders_p set amount = amount + ?, status = ? where tenant = ? and order_id = ?';

set @delta=5.25,@new_status='paid',@t=2,@oid=202;
execute s_upd using @delta,@new_status,@t,@oid;

set @delta='1.00',@new_status='closed',@t='3',@oid='301';
execute s_upd using @delta,@new_status,@t,@oid;

select tenant,order_id,status,amount from orders_p order by tenant,order_id;
```

### MySQL result

```text
tenant order_id status amount
2 202 paid 45.25
3 301 closed 51.00
```

### MO result

The first numeric-parameter execution succeeds. The second execution, using string variables for the partition/key predicates, fails:

```text
ERROR 20101 (HY000): internal error: panic BUG:
github.com/matrixorigin/matrixone/pkg/sql/colexec/multi_update.(*PartitionMultiUpdate).getPartitionIndex
pkg/sql/colexec/multi_update/multi_update_partition.go:387
github.com/matrixorigin/matrixone/pkg/sql/colexec/multi_update.(*PartitionMultiUpdate).writeTable.func1
pkg/sql/colexec/multi_update/multi_update_partition.go:192
github.com/matrixorigin/matrixone/pkg/partitionservice.PruneResult
```

The row is not updated:

```text
tenant order_id status amount
2 202 paid 45.25
3 301 paid 50.00
```

### Notes

The same prepared update on a partitioned table **without** secondary indexes did not reproduce the panic. The secondary indexes appear to be part of the trigger condition.

### Expected

Prepared `UPDATE` should update the matching partitioned row and maintain secondary indexes, even when parameter values are supplied as string user variables that MySQL coerces to the target numeric key type.