Enter signature scope in SerializeReturnTypeForSignature

[jakebailey]

Fixes #3570

[Copilot]

Pull request overview

This PR fixes a declaration emit regression (issue #3570) where type parameters could be renamed/serialized in a way that produced invalid .d.ts output (e.g., references to an undefined T_1). The fix ensures the node builder enters the correct signature scope when serializing a signature’s return type, aligning output with the TypeScript submodule baselines.

Changes:

• Introduce enterSignatureScope helper and reuse it for signature-to-declaration serialization.
• Enter signature scope in NodeBuilder.SerializeReturnTypeForSignature so return type serialization sees the correct type parameter/parameter scope.
• Update/clean up submodule baseline expectations, including removing now-unnecessary .diff files.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/baselines/reference/submodule/conformance/typeParametersAvailableInNestedScope3.js.diff	Removed baseline diff now that output matches the TypeScript submodule reference.
testdata/baselines/reference/submodule/conformance/typeParametersAvailableInNestedScope3.js	Updated expected output to the now-correct generic parameter naming/visibility behavior.
testdata/baselines/reference/submodule/compiler/declarationEmitShadowing.js.diff	Removed baseline diff after fixing incorrect type parameter reference that caused TS2304.
testdata/baselines/reference/submodule/compiler/declarationEmitShadowing.js	Updated expected .d.ts output to avoid undefined type parameter references.
testdata/baselines/reference/submodule/compiler/declarationEmitNestedGenerics.js.diff	Removed baseline diff now that nested generic naming matches the submodule reference.
testdata/baselines/reference/submodule/compiler/declarationEmitNestedGenerics.js	Updated expected nested generic signature output to the corrected naming.
internal/checker/nodebuilderscopes.go	Added enterSignatureScope helper to centralize entering signature scope.
internal/checker/nodebuilderimpl.go	Updated signature-to-declaration serialization to use enterSignatureScope.
internal/checker/nodebuilder.go	Updated SerializeReturnTypeForSignature to enter signature scope before serializing the return type.