Don't defer index types for non-generic substitution types

[Andarist]

fixes #61728

[Copilot]

Pull Request Overview

This PR ensures that index types on non-generic substitution types are resolved immediately (instead of deferred), addressing #61728.

• Added two new compiler tests to verify index lookups on non-generic substitution types.
• Modified getIndexType in checker.ts to bypass deferral when both baseType and constraint are non-generic.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

File	Description
tests/cases/compiler/substitutionTypeNonGenericIndexType1.ts	Adds a test for boolean conditional index resolution on non-generic substitution types.
tests/cases/compiler/substitutionTypeNonGenericIndexType2.ts	Adds a test for infer-based index resolution on non-generic substitution types.
src/compiler/checker.ts	Updates getIndexType to immediately compute index types for non-generic substitutions.

Comments suppressed due to low confidence (3)

tests/cases/compiler/substitutionTypeNonGenericIndexType2.ts:1

• [nitpick] Consider adding a reference to GitHub issue #61728 at the top of this test for consistency with related test files.

// @strict: true

tests/cases/compiler/substitutionTypeNonGenericIndexType2.ts:14

• [nitpick] Consider adding additional test cases where either the baseType or the constraint is generic to ensure this new index type logic covers mixed generic scenarios.

const test: Test["rejectClose"] = "rejectClose";