fix(analysis): assign AstNode to the scope a node introduces

[keshav9926]

Description

Scope.AstNode is documented as "the AST node that introduces this scope into the scope tree", but buildScopeTree assigned it on the parent scope rather than on the scope the node actually introduces:

nextScope = NewScope(scope)
scopeOfNode[node] = nextScope
if scope != nil {
    scope.Children = append(scope.Children, nextScope)
    scope.AstNode = node   // <- parent's field, not nextScope's
}

Two things follow from that:

• a newly created scope's own AstNode was never assigned, so every scope in the tree reported nil except the synthetic root
• a parent's AstNode was reassigned by each scope-creating child in turn, so it ended up holding whichever child was visited last

Reproduction

For this source:

let x = 1
function f() {
    let y = 2
}
{
    let z = 3
}

walking ScopeOfNode on master gives:

scope introduced by	AstNode reported	expected
program	statement_block	program
function_declaration	statement_block (its own body)	function_declaration
statement_block	nil	statement_block
statement_block	nil	statement_block

Fix

Assign nextScope.AstNode = node. ScopeOfNode already mapped each node to the correct scope, so GetScope was never affected and nothing that relies on it changes behaviour — this only makes the field match its documented meaning.

I found this while reading ScopeBuilder to understand how scope resolution works before writing a checker. The field is exported on an exported type, so checker authors reaching for scope.AstNode currently get nil or the wrong node; that seemed worth fixing even though nothing in-tree reads it today.

Type of change

• Bug fix
• New feature/enhancement
• New checker
• Documentation update
• Other (please describe)

Checklist

• I have updated the documentation, if applicable
• All new checkers are in the checkers directory
• I have added tests, if applicable
• I have tested my changes (run make testall)
• Checker YAML files follow the required format
• CI checks are passing

Test_ScopeAstNode walks ScopeOfNode and asserts every scope points at its own introducing node, with explicit subtests for the parent-overwrite and function-vs-body cases. It fails on master with:

scope introduced by "program" points at "statement_block" instead
scope introduced by "function_declaration" points at "statement_block" instead
scope introduced by "statement_block" has a nil AstNode

and passes with the fix.

On my machine (Windows) TestFindYamlTestFiles fails both before and after this change, so it looks unrelated and pre-existing rather than something this PR introduces — happy to look into it separately if that's useful.

Related Issues

None.

Additional Notes

This PR was developed with AI assistance (Claude); the bug was found by reading the scope builder, and the fix and tests were reviewed and verified locally before submission.

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