fix: explain ast of multi table insert stmt

[dantengsky]

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Summary

fix explain ast insert first/all.

before this PR, explain ast of multiple table insert stmt panics, with this PR:

query T
EXPLAIN AST
INSERT FIRST
WHEN order_amount > 1000 THEN INTO cat.db1.t1(order_id, update_note) VALUES (order_id, 'PriorityHandling')
WHEN order_amount > 500 THEN INTO cat2.db2.t2 VALUES (order_id, 'ExpressHandling')
WHEN order_amount > 100 THEN
   INTO cat3.db3.t3 VALUES (order_id, 'StandardHandling')
   INTO cat3.db3.t4 VALUES (order_id, 'StandardHandling')
ELSE INTO processing_updates VALUES (order_id, 'ReviewNeeded')
SELECT order_id, order_amount FROM orders_placed;
----
INSERT FIRST (children 5)
├── WHEN (children 2)
│   ├── Function > (children 2)
│   │   ├── ColumnIdentifier order_amount
│   │   └── Literal UInt64(1000)
│   └── INTO (children 2)
│       ├── TableIdentifier cat.db1.t1
│       └── Columns (children 2)
│           ├── Identifier order_id
│           └── Identifier update_note
├── WHEN (children 2)
│   ├── Function > (children 2)
│   │   ├── ColumnIdentifier order_amount
│   │   └── Literal UInt64(500)
│   └── INTO (children 1)
│       └── TableIdentifier cat2.db2.t2
├── WHEN (children 3)
│   ├── Function > (children 2)
│   │   ├── ColumnIdentifier order_amount
│   │   └── Literal UInt64(100)
│   ├── INTO (children 1)
│   │   └── TableIdentifier cat3.db3.t3
│   └── INTO (children 1)
│       └── TableIdentifier cat3.db3.t4
├── ELSE (children 1)
│   └── INTO (children 1)
│       └── TableIdentifier processing_updates
└── Query (children 1)
    └── QueryBody (children 1)
        └── SelectQuery (children 2)
            ├── SelectList (children 2)
            │   ├── Target (children 1)
            │   │   └── ColumnIdentifier order_id
            │   └── Target (children 1)
            │       └── ColumnIdentifier order_amount
            └── TableList (children 1)
                └── TableIdentifier orders_placed

• Fixes #[Link the issue here]

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[SkyFan2002]

LGTM. Thanks!