fix(messages): allow guard-to-guard chat

[LoopyB]

Summary

Fixed guard-to-guard messaging in the backend.

Previously, sendMessage only allowed guard-to-employer and employer-to-guard communication, which caused guard-to-guard chat to be rejected with a 403 error.

Changes

• Updated message role validation to allow:
  • guard → guard
  • guard → employer
  • employer → guard
• Kept employer → employer and admin-involved messaging blocked
• Updated the Swagger messaging tag description to match the new allowed communication paths
• Added a focused regression test for message role validation

Validation

Passed:

npm test -- --runInBand message.controller.test.js