Support compound field access after subscripts, e.g. payload[1].a

## Description

DataFusion accepts compound field access like:

```sql
SELECT payload.a[1]
FROM t
```

but rejects:

```sql
SELECT payload[1].a
FROM t
```

and reports error 'Dot access not supported for non-string expr', even though both represent valid nested field access patterns and both are parsed by `sqlparser`
as `Expr::CompoundFieldAccess`.

## AST Shape

In `sqlparser`, both expressions are represented as `Expr::CompoundFieldAccess`.

### `payload.a[1]`

```text
Expr::CompoundFieldAccess
├── root: Identifier("payload")
└── access_chain:
1. Dot(Identifier("a"))
2. Subscript(Index(Value(Number("1"))))
```

Intended logical chain:

```text
expr0 = Column("payload")
expr1 = GetField(expr0, NamedStructField("a"))
expr2 = GetField(expr1, ListIndex(1))
```

### `payload[1].a`

```text
Expr::CompoundFieldAccess
├── root: Identifier("payload")
└── access_chain:
1. Subscript(Index(Value(Number("1"))))
2. Dot(Identifier("a"))
```

Intended logical chain:

```text
expr0 = Column("payload")
expr1 = GetField(expr0, ListIndex(1))
expr2 = GetField(expr1, NamedStructField("a"))
```

## Current Behavior

In `sql_compound_field_access_to_expr`, dot access currently accepts only a string literal form:

```text
AccessExpr::Dot(SQLExpr::Value(SingleQuotedString | DoubleQuotedString))
```

and rejects:

```text
AccessExpr::Dot(SQLExpr::Identifier(_))
```

As a result, `payload[1].a` fails to plan even though the field name `a` is statically known.

## Expected Behavior

`payload[1].a` should be accepted and planned the same way as other named struct field accesses,
producing a `GetFieldAccess::NamedStructField` step after the subscript access.

In other words, these should both be supported:

- `payload.a[1]`
- `payload[1].a`

## Why This Matters

Downstream systems currently need SQL AST rewrites to convert `payload[1].a` into a form that
DataFusion accepts internally. Supporting this directly in DataFusion would remove that
workaround and make nested field access behavior more consistent.