Fix #30408: Fallback to string index signature for literal and generic indexing

[mhughes2012]

Fixes #30408

Description

This PR addresses issue #30408 where indexing an object with a string literal or a generic type parameter constrained to string would fail to fall back to an available string index signature if a specific property was not found. This often resulted in any types or confusing "property missing" errors even when a valid index signature existed.

The change modifies the property lookup logic in src/compiler/checker.ts to:

1. First, check for a specific named property (preserving existing priority).
2. Second, if no specific property is found, check if the index type is assignable to string.
3. If assignable, attempt to resolve the type via the string index signature.

Checklist

• There is an associated issue in the Backlog milestone
• Code is up-to-date with the main branch
• You've successfully run hereby runtests locally
• There are new or updated unit tests validating the change

Test Case

I have added a new compiler test case tests/cases/compiler/stringLiteralIndexingWithIndexSignature.ts which validates:

• Literal indexing on a type with a string index signature.
• Generic indexing (K extends string) correctly resolving to the index signature's type.
• Verification that reference baselines correctly capture these inferred types.

[mhughes2012]

@microsoft-github-policy-service agree