Get connected user's info by participant

[lnqminh3003]

Summary

• Add GET /api/participantConnections/connected-users endpoint to fetch user profile info for all participants connected with a given participant in an event
• Returns user profile data (name, email, linkedinUrl, bio, profilePhoto, socialLinks), participant info, and connection description for each connection
• Returns all connections including multiple connections to the same user with different descriptions (no dedup)
• Uses Mongoose .populate() to resolve user data in a single query instead of separate lookups
• Returns 200 with empty array when no connections exist (not 404)
• Handles null userId gracefully (guest participants) — returns user: null with participant info still included
• Restores userId: Schema.Types.ObjectId | null on IParticipant interface to match the Mongoose schema's default: null
• Updates API_REFERENCE.md with full endpoint documentation

Endpoint

GET /api/participantConnections/connected-users?eventId=<id>&participantId=<id>
Authorization: Bearer <token>

Response (200):

[
  {
    "user": { "_id": "...", "name": "...", "email": "...", ... },
    "participantId": "...",
    "participantName": "...",
    "connectionDescription": "Both love hiking"
  }
]