[Refactor] Add new AST node types and resolve AST TODOs

[colinthebomb1]

Overview:

This PR adds new node types and adds functionality to existing ones to improve the constructed ASTs for test files, resolving most of the TODOs identified in PR #97.

Code Changes:

• Added TypeNode for SQL type keywords (TEXT, DATE, NULL, etc.) — replaces FunctionNode("TEXT", _args=[LiteralNode("{}")])
• Added ListNode for value lists (e.g. the RHS of IN expressions) — replaces raw Python lists
• Added IntervalNode for INTERVAL expressions — replaces FunctionNode("INTERVAL", ...)
• Added CaseNode for CASE WHEN ... THEN ... ELSE ... END — replaces nested FunctionNode("CASE"/"WHEN"/"THEN"/"ELSE")
• Added _distinct and _distinct_on parameters to SelectNode for SELECT DISTINCT / DISTINCT ON
• Added NodeType.TYPE, LIST, INTERVAL, CASE to enums
• Updated expected ASTs in data/asts.py to use the new node types

[Copilot]

Pull request overview

This PR refactors the internal AST representation to better model several SQL constructs (types/keywords, value lists, intervals, CASE expressions, and DISTINCT variants) and updates the expected AST fixtures accordingly.

Changes:

• Introduces new AST node types: TypeNode, ListNode, IntervalNode, and CaseNode, plus new NodeType enum values.
• Extends SelectNode to represent DISTINCT and DISTINCT ON.
• Updates data/asts.py expected ASTs (and tweaks formatter tests) to use the new node types.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
core/ast/node.py	Adds new node classes and extends SelectNode to model DISTINCT/DISTINCT ON.
core/ast/enums.py	Adds NodeType enum values for the new node types.
data/asts.py	Updates expected AST fixtures to use TypeNode, ListNode, IntervalNode, CaseNode, and DISTINCT metadata.
tests/test_query_formatter.py	Adjusts/relaxes formatter tests by commenting out some formatter calls and minor assertion key change.

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[Copilot]

SelectNode tracks _distinct_on but does not include it in children. Any generic AST traversal that relies on children will miss DISTINCT ON expressions, which can lead to incorrect rewrites/formatting/analysis. Consider representing DISTINCT ON as part of the node’s subtree (e.g., include it in children with a dedicated wrapper node/field-aware traversal) and update consumers accordingly.

Suggested change

	super().__init__(NodeType.SELECT, children=_items, **kwargs)
	# Include DISTINCT ON expressions in children so generic AST traversals see them.
	children: List[Node] = list(_items) if _items is not None else []
	if _distinct_on is not None:
	children.append(_distinct_on)
	super().__init__(NodeType.SELECT, children=children, **kwargs)