Fixed mapped type modifier propagation when remapping multiple keys to same property

[heathdutton]

Fixes#62318

[Copilot]

Pull request overview

This PR fixes a bug where mapped types with key remapping would produce inconsistent modifier propagation when multiple keys remapped to the same output property name. The behavior was dependent on the order of keys in the source type, which was incorrect.

Key changes:

• Added logic to merge optional and readonly modifiers when multiple source keys map to the same target property in mapped types
• Added comprehensive test coverage for both optional and readonly modifier merging scenarios

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/compiler/checker.ts	Added logic in addMemberForKeyTypeWorker to merge modifiers (optional/readonly) when an existing property is encountered during mapped type key remapping
tests/cases/compiler/mappedTypeRemappingModifierMerging.ts	New test case verifying consistent modifier propagation when multiple keys remap to the same property, testing both optional and readonly modifiers
tests/baselines/reference/mappedTypeRemappingModifierMerging.types	Baseline file showing expected type inference for the new test
tests/baselines/reference/mappedTypeRemappingModifierMerging.symbols	Baseline file showing expected symbol resolution for the new test
tests/baselines/reference/mappedTypeRemappingModifierMerging.js	Baseline file showing expected JavaScript output and declaration files
tests/baselines/reference/mappedTypeRemappingModifierMerging.errors.txt	Baseline file showing expected readonly property assignment errors

[heathdutton]

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