Split ast.go into one file per AST node

Refactor the ast package to have each AST node type in its own file:
- node.go: Node interface
- script.go, batch.go: Root types
- statement.go, select_statement.go: Statement types
- query_expression.go, query_specification.go: Query types
- select_element.go, select_scalar_expression.go, select_star_expression.go
- scalar_expression.go, column_reference_expression.go
- integer_literal.go, string_literal.go, function_call.go
- identifier.go, multi_part_identifier.go, identifier_or_value_expression.go
- from_clause.go, table_reference.go, named_table_reference.go, qualified_join.go
- schema_object_name.go
- where_clause.go, boolean_expression.go, boolean_comparison_expression.go
- boolean_binary_expression.go
- group_by_clause.go, grouping_specification.go, expression_grouping_specification.go
- having_clause.go, order_by_clause.go, expression_with_sort_order.go
- optimizer_hint.go

Also update metadata.json to use empty object instead of skip: false.

Author: claude

ast/ast.go

@@ -0,0 +1,8 @@
+package ast
+
+// Batch represents a T-SQL batch of statements.
+type Batch struct {
+	Statements []Statement `json:"Statements,omitempty"`
+}
+
+func (*Batch) node() {}

ast/batch.go

@@ -0,0 +1,11 @@
+package ast
+
+// BooleanBinaryExpression represents a binary boolean expression (AND, OR).
+type BooleanBinaryExpression struct {
+	BinaryExpressionType string            `json:"BinaryExpressionType,omitempty"`
+	FirstExpression      BooleanExpression `json:"FirstExpression,omitempty"`
+	SecondExpression     BooleanExpression `json:"SecondExpression,omitempty"`
+}
+
+func (*BooleanBinaryExpression) node()              {}
+func (*BooleanBinaryExpression) booleanExpression() {}

ast/boolean_binary_expression.go

@@ -0,0 +1,11 @@
+package ast
+
+// BooleanComparisonExpression represents a comparison expression.
+type BooleanComparisonExpression struct {
+	ComparisonType   string           `json:"ComparisonType,omitempty"`
+	FirstExpression  ScalarExpression `json:"FirstExpression,omitempty"`
+	SecondExpression ScalarExpression `json:"SecondExpression,omitempty"`
+}
+
+func (*BooleanComparisonExpression) node()              {}
+func (*BooleanComparisonExpression) booleanExpression() {}

ast/boolean_comparison_expression.go

@@ -0,0 +1,7 @@
+package ast
+
+// BooleanExpression is the interface for boolean expressions.
+type BooleanExpression interface {
+	Node
+	booleanExpression()
+}

ast/boolean_expression.go

@@ -0,0 +1,10 @@
+package ast
+
+// ColumnReferenceExpression represents a column reference.
+type ColumnReferenceExpression struct {
+	ColumnType          string               `json:"ColumnType,omitempty"`
+	MultiPartIdentifier *MultiPartIdentifier `json:"MultiPartIdentifier,omitempty"`
+}
+
+func (*ColumnReferenceExpression) node()             {}
+func (*ColumnReferenceExpression) scalarExpression() {}

ast/column_reference_expression.go

@@ -0,0 +1,10 @@
+package ast
+
+// ExpressionGroupingSpecification represents a grouping by expression.
+type ExpressionGroupingSpecification struct {
+	Expression              ScalarExpression `json:"Expression,omitempty"`
+	DistributedAggregation bool             `json:"DistributedAggregation,omitempty"`
+}
+
+func (*ExpressionGroupingSpecification) node()                  {}
+func (*ExpressionGroupingSpecification) groupingSpecification() {}

ast/expression_grouping_specification.go

@@ -0,0 +1,9 @@
+package ast
+
+// ExpressionWithSortOrder represents an expression with sort order.
+type ExpressionWithSortOrder struct {
+	SortOrder  string           `json:"SortOrder,omitempty"`
+	Expression ScalarExpression `json:"Expression,omitempty"`
+}
+
+func (*ExpressionWithSortOrder) node() {}

ast/expression_with_sort_order.go

@@ -0,0 +1,8 @@
+package ast
+
+// FromClause represents a FROM clause.
+type FromClause struct {
+	TableReferences []TableReference `json:"TableReferences,omitempty"`
+}
+
+func (*FromClause) node() {}

ast/from_clause.go

@@ -0,0 +1,12 @@
+package ast
+
+// FunctionCall represents a function call.
+type FunctionCall struct {
+	FunctionName     *Identifier        `json:"FunctionName,omitempty"`
+	Parameters       []ScalarExpression `json:"Parameters,omitempty"`
+	UniqueRowFilter  string             `json:"UniqueRowFilter,omitempty"`
+	WithArrayWrapper bool               `json:"WithArrayWrapper,omitempty"`
+}
+
+func (*FunctionCall) node()             {}
+func (*FunctionCall) scalarExpression() {}