feat: enhance messaging system with improved API and tests

- Refactor messaging API endpoints for admin, shop, and vendor
- Add message-specific exceptions (ConversationNotFoundException, etc.)
- Enhance messaging service with additional helper methods
- Add comprehensive test fixtures for messaging
- Add integration tests for admin and vendor messaging APIs
- Add unit tests for messaging and attachment services

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/messaging_service.py

@@ -567,6 +567,118 @@ class MessagingService:
         db.flush()
         return result > 0
 
+    # =========================================================================
+    # RECIPIENT QUERIES
+    # =========================================================================
+
+    def get_vendor_recipients(
+        self,
+        db: Session,
+        vendor_id: int | None = None,
+        search: str | None = None,
+        skip: int = 0,
+        limit: int = 50,
+    ) -> tuple[list[dict], int]:
+        """
+        Get list of vendor users as potential recipients.
+
+        Args:
+            db: Database session
+            vendor_id: Optional vendor ID filter
+            search: Search term for name/email
+            skip: Pagination offset
+            limit: Max results
+
+        Returns:
+            Tuple of (recipients list, total count)
+        """
+        from models.database.vendor import VendorUser
+
+        query = (
+            db.query(User, VendorUser)
+            .join(VendorUser, User.id == VendorUser.user_id)
+            .filter(User.is_active == True)  # noqa: E712
+        )
+
+        if vendor_id:
+            query = query.filter(VendorUser.vendor_id == vendor_id)
+
+        if search:
+            search_pattern = f"%{search}%"
+            query = query.filter(
+                (User.username.ilike(search_pattern))
+                | (User.email.ilike(search_pattern))
+                | (User.first_name.ilike(search_pattern))
+                | (User.last_name.ilike(search_pattern))
+            )
+
+        total = query.count()
+        results = query.offset(skip).limit(limit).all()
+
+        recipients = []
+        for user, vendor_user in results:
+            name = f"{user.first_name or ''} {user.last_name or ''}".strip() or user.username
+            recipients.append({
+                "id": user.id,
+                "type": ParticipantType.VENDOR,
+                "name": name,
+                "email": user.email,
+                "vendor_id": vendor_user.vendor_id,
+                "vendor_name": vendor_user.vendor.name if vendor_user.vendor else None,
+            })
+
+        return recipients, total
+
+    def get_customer_recipients(
+        self,
+        db: Session,
+        vendor_id: int | None = None,
+        search: str | None = None,
+        skip: int = 0,
+        limit: int = 50,
+    ) -> tuple[list[dict], int]:
+        """
+        Get list of customers as potential recipients.
+
+        Args:
+            db: Database session
+            vendor_id: Optional vendor ID filter (required for vendor users)
+            search: Search term for name/email
+            skip: Pagination offset
+            limit: Max results
+
+        Returns:
+            Tuple of (recipients list, total count)
+        """
+        query = db.query(Customer).filter(Customer.is_active == True)  # noqa: E712
+
+        if vendor_id:
+            query = query.filter(Customer.vendor_id == vendor_id)
+
+        if search:
+            search_pattern = f"%{search}%"
+            query = query.filter(
+                (Customer.email.ilike(search_pattern))
+                | (Customer.first_name.ilike(search_pattern))
+                | (Customer.last_name.ilike(search_pattern))
+            )
+
+        total = query.count()
+        results = query.offset(skip).limit(limit).all()
+
+        recipients = []
+        for customer in results:
+            name = f"{customer.first_name or ''} {customer.last_name or ''}".strip()
+            recipients.append({
+                "id": customer.id,
+                "type": ParticipantType.CUSTOMER,
+                "name": name or customer.email,
+                "email": customer.email,
+                "vendor_id": customer.vendor_id,
+            })
+
+        return recipients, total
+
 
 # Singleton instance
 messaging_service = MessagingService()