fix: restore JSON backwards compatibility for .roomodes files

[daniel-lxs]

Summary

Fixes JSON backwards compatibility issue introduced in #5099.

Changes

• Add JSON fallback to parseYamlSafely for reading old JSON .roomodes files
• Remove defaultStringType: 'PLAIN' from all yaml.stringify calls to ensure JSON-compatible output

Testing

All existing tests pass (18/18).

Related Issues

Closes #5180
Closes #5139

---

Important

Adds JSON fallback for .roomodes files and ensures JSON-compatible YAML output by removing defaultStringType: 'PLAIN'.

• Behavior:
  • Adds JSON fallback in parseYamlSafely() in CustomModesManager.ts for .roomodes files.
  • Removes defaultStringType: 'PLAIN' from yaml.stringify calls in CustomModesManager.ts, SimpleInstaller.ts, and migrateSettings.ts for JSON-compatible output.
• Testing:
  • All existing tests pass (18/18).

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[delve-auditor]

✅ No security or compliance issues detected. Reviewed everything up to ea6bc15.

Security Overview

• 🔎 Scanned files: 3 changed file(s)

Detected Code Changes

Change Type	Relevant files
Bug Fix	► CustomModesManager.ts     Add JSON fallback for parsing old .roomodes files     Remove defaultStringType: 'PLAIN' from yaml.stringify ► SimpleInstaller.ts     Remove defaultStringType: 'PLAIN' from yaml.stringify ► migrateSettings.ts     Remove defaultStringType: 'PLAIN' from yaml.stringify

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