Fix maybeMappedType function

[ahejlsberg]

Fixes #3509.

[Copilot]

Pull request overview

Fixes the TypeScript-Go checker’s mapped-type “Did you mean P in K?” suggestion so that TS2690 is reported (with the suggestion) instead of falling back to the generic TS2693, aligning output with the upstream TypeScript baselines and resolving #3509.

Changes:

• Fix Checker.maybeMappedType to walk up from the identifier to the computed property/type literal context before deciding to emit the mapped-type suggestion.
• Tighten the key-type check for the suggestion using allTypesAssignableToKindEx(..., strict=true) so any/unknown/null/undefined don’t incorrectly qualify.
• Update/clean up baselines and triage tracking now that the output matches the submodule expectations.

Reviewed changes

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

File	Description
internal/checker/checker.go	Corrects ancestor-walk logic in maybeMappedType and refines the “is union of string/number literals” check to restore mapped-type suggestion diagnostics.
testdata/submoduleTriaged.txt	Removes the triage entry for #3509 since it’s no longer a known baseline mismatch.
testdata/baselines/reference/submoduleTriaged/compiler/typeUsedAsTypeLiteralIndex.errors.txt.diff	Deletes the triaged diff baseline because the mismatch is resolved.
testdata/baselines/reference/submodule/compiler/typeUsedAsTypeLiteralIndex.errors.txt	Updates the reference baseline to reflect the restored TS2690 + suggestion output.