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/exceptions/__init__.py

@@ -103,8 +103,11 @@ from .inventory import (
 
 # Message exceptions
 from .message import (
+    AttachmentNotFoundException,
     ConversationClosedException,
     ConversationNotFoundException,
+    InvalidConversationTypeException,
+    InvalidRecipientTypeException,
     MessageAttachmentException,
     MessageNotFoundException,
     UnauthorizedConversationAccessException,
@@ -389,4 +392,7 @@ __all__ = [
     "ConversationClosedException",
     "MessageAttachmentException",
     "UnauthorizedConversationAccessException",
+    "InvalidConversationTypeException",
+    "InvalidRecipientTypeException",
+    "AttachmentNotFoundException",
 ]