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/api/v1/shop/messages.py

@@ -21,7 +21,12 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_customer_api
 from app.core.database import get_db
-from app.exceptions import ConversationNotFoundException, VendorNotFoundException
+from app.exceptions import (
+    AttachmentNotFoundException,
+    ConversationClosedException,
+    ConversationNotFoundException,
+    VendorNotFoundException,
+)
 from app.services.message_attachment_service import message_attachment_service
 from app.services.messaging_service import messaging_service
 from models.database.customer import Customer
@@ -292,12 +297,7 @@ async def send_message(
 
     # Check if conversation is closed
     if conversation.is_closed:
-        from fastapi import HTTPException
-
-        raise HTTPException(
-            status_code=400,
-            detail="Cannot send messages to a closed conversation",
-        )
+        raise ConversationClosedException(conversation_id)
 
     # Process attachments
     attachment_data = []
@@ -405,7 +405,6 @@ async def download_attachment(
 
     Validates that customer has access to the conversation.
     """
-    from fastapi import HTTPException
     from fastapi.responses import FileResponse
 
     vendor = getattr(request.state, "vendor", None)
@@ -433,7 +432,7 @@ async def download_attachment(
     )
 
     if not attachment:
-        raise HTTPException(status_code=404, detail="Attachment not found")
+        raise AttachmentNotFoundException(attachment_id)
 
     return FileResponse(
         path=attachment.file_path,
@@ -455,7 +454,6 @@ async def get_attachment_thumbnail(
 
     Validates that customer has access to the conversation.
     """
-    from fastapi import HTTPException
     from fastapi.responses import FileResponse
 
     vendor = getattr(request.state, "vendor", None)
@@ -483,7 +481,7 @@ async def get_attachment_thumbnail(
     )
 
     if not attachment or not attachment.thumbnail_path:
-        raise HTTPException(status_code=404, detail="Thumbnail not found")
+        raise AttachmentNotFoundException(f"{attachment_id}/thumbnail")
 
     return FileResponse(
         path=attachment.thumbnail_path,