fix: report invalid Claude settings JSON

[yoldaolmak]

Summary

• distinguish JSON-shaped settings input from a settings-file path
• report invalid inline JSON, settings input files, and existing Claude settings distinctly
• await and strengthen the asynchronous error-path tests

This addresses the Claude settings JSON validation portion of #242. GitHub Actions workflow YAML is parsed by GitHub before the action runs, so it remains outside this action-level change.

Verification

• bun test (801 passing)
• bun run typecheck
• bun run format:check