[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 DataTypeNode for SQL data types used in CAST expressions (TEXT, DATE, INTEGER, etc.)
• Added TimeUnitNode for SQL time units used in INTERVAL and temporal functions (DAY, SECOND, MONTH, etc.)
• 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 WhenThenNode for individual WHEN/THEN branches within a CASE expression
• Added CaseNode for CASE WHEN ... THEN ... ELSE ... END — uses WhenThenNode instead of raw Python tuples
• Added _distinct and _distinct_on parameters to SelectNode for SELECT DISTINCT / DISTINCT ON
• SQL NULL represented as LiteralNode(None) rather than a separate node type
• Added NodeType.DATA_TYPE, TIME_UNIT, LIST, INTERVAL, CASE, WHEN_THEN 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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[HazelYuAhiru]

Overall looks good to me! Just to confirm, are we now using a more specific approach (creating specific new node types when they are encountered, such as IntervalNode and WhenThenNode) rather than a more general approach for handling new queries? Also, should we add code in ast_util to visualize the new nodes as well?

[baiqiushi]

Good refactoring overall — replacing FunctionNode workarounds with proper typed AST nodes is a solid improvement. CI is green. A few issues to address:

Bug: SelectNode._distinct_on is not included in children, which breaks generic AST traversals.

Suggestion: The hardcoded allowlists in DataTypeNode and TimeUnitNode are quite restrictive and will throw ValueError on valid SQL types/units not in the set.

[baiqiushi]

Thanks a lot @colinthebomb1 for addressing the comments!