planner: disambiguate MPP dynamic partition pruning in explain output

## Enhancement

When `EXPLAIN` prints dynamic partition pruning information for an MPP plan, the partition information is attached to the root `PhysicalTableReader` access object. For an MPP plan tree that contains multiple table scans of the same table name, this output can become ambiguous.

Example:

```sql
set tidb_allow_mpp = 1;
set tidb_enforce_mpp = 1;
set tidb_partition_prune_mode = dynamic;

create table t (a int, b char(20)) partition by range(a) (
partition p0 values less than (2),
partition p1 values less than (3),
partition pMax values less than maxvalue
);
alter table t set tiflash replica 1;

explain format = 'plan_tree'
select *
from (select * from t where a = 1) x
join (select * from t where a = 2) y on x.b = y.b;
```

The plan can look like this:

```text
TableReader root partition:p0 of t, partition:p1 of t MppVersion: 3, data:ExchangeSender
└─ExchangeSender mpp[tiflash] ExchangeType: PassThrough
└─HashJoin mpp[tiflash] inner join, equal:[eq(test.t.b, test.t.b)]
├─ExchangeReceiver(Build) mpp[tiflash]
│ └─ExchangeSender mpp[tiflash] ExchangeType: Broadcast, Compression: FAST
│ └─Selection mpp[tiflash] not(isnull(test.t.b))
│ └─TableFullScan mpp[tiflash] table:t pushed down filter:eq(test.t.a, 1), keep order:false, stats:pseudo, PartitionTableScan:true
└─Selection(Probe) mpp[tiflash] not(isnull(test.t.b))
└─TableFullScan mpp[tiflash] table:t pushed down filter:eq(test.t.a, 2), keep order:false, stats:pseudo, PartitionTableScan:true
```

The root access object says `partition:p0 of t, partition:p1 of t`, while both child scans are printed as `table:t`. For a larger MPP tree, or when the same base table appears many times, it is hard to tell which partition pruning result belongs to which scan without reverse-engineering the predicates and tree shape.

It would be helpful if the dynamic partition pruning output for multi-scan MPP `TableReader` plans included a stable disambiguator, for example:

- the corresponding table scan plan id, such as `partition:p0 of TableFullScan_x`;
- the effective table alias when available;
- or another scan-local identifier that can be matched back to the child scan in the plan tree.

Related background: #30365 fixed/covered the older problem where MPP dynamic partition pruning information was not shown in `EXPLAIN` at all. This issue is about the remaining usability problem when multiple collected MPP table scans have the same printed table name.