Fixed mapped types not being considered as homomorphic with substitution constraints

[Andarist]

fixes microsoft/TypeScript#63132

[Copilot]

Pull request overview

This PR fixes a TypeScript checker behavior in the Go port so that mapped types remain homomorphic even when their keyof T constraint flows through a substitution type (matching the regression described in microsoft/TypeScript#63132). It also adds a targeted compiler regression test and commits the corresponding reference baselines.

Changes:

• Unwrap substitution types when detecting a homomorphic mapped type variable, so keyof T extends ... ? { [K in keyof T]: ... } : ... continues to be recognized as homomorphic.
• Add a new compiler test case reproducing the scenario from the linked upstream issue.
• Add committed .types and .symbols reference baselines for the new test.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
internal/checker/checker.go	Adjusts homomorphic mapped type detection to use the “actual” (unsubstituted) type variable.
testdata/tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts	New regression test covering homomorphic behavior through conditional wrapping.
testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.types	Reference type baseline for the new test.
testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.symbols	Reference symbol baseline for the new test.