JS: Avoid overriding Expr predicates in xUnit.qll

[asgerf]

The XUnit family of library models contained an override of getFirstToken and getLastToken. Overriding the common definitions was not actually necessary, and induced some dependencies that are problematic for overlay locality.

This PR removes the problematic dependency by introducing separate root-defs for the custom interpretation of these predicates.

[Copilot]

Pull Request Overview

This PR removes overridden implementations of getFirstToken and getLastToken predicates in the xUnit.qll library to avoid problematic dependencies for overlay locality. The change introduces separate root definitions for custom interpretations of these predicates instead of overriding common definitions.

• Introduces a FinalExpr type alias to avoid inheritance conflicts
• Adds explicit getFirstToken and getLastToken methods to BracketedListOfExpressions class
• Refactors ArrayExprIsABracketedListOfExpressions to use instance-of syntax instead of direct inheritance

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
javascript/ql/lib/semmle/javascript/frameworks/xUnit.qll	Refactored inheritance structure and added explicit token accessor methods
javascript/ql/test/library-tests/frameworks/xUnit/tests.expected	Updated test expectations to reflect changes in token positioning