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",
 ]

app/exceptions/message.py

@@ -61,3 +61,40 @@ class UnauthorizedConversationAccessException(BusinessLogicException):
             error_code="CONVERSATION_ACCESS_DENIED",
             details={"conversation_id": conversation_id},
         )
+
+
+class InvalidConversationTypeException(ValidationException):
+    """Raised when conversation type is not valid for the operation."""
+
+    def __init__(self, message: str, allowed_types: list[str] | None = None):
+        super().__init__(
+            message=message,
+            error_code="INVALID_CONVERSATION_TYPE",
+            details={"allowed_types": allowed_types} if allowed_types else None,
+        )
+
+
+class InvalidRecipientTypeException(ValidationException):
+    """Raised when recipient type doesn't match conversation type."""
+
+    def __init__(self, conversation_type: str, expected_recipient_type: str):
+        super().__init__(
+            message=f"{conversation_type} conversations require a {expected_recipient_type} recipient",
+            error_code="INVALID_RECIPIENT_TYPE",
+            details={
+                "conversation_type": conversation_type,
+                "expected_recipient_type": expected_recipient_type,
+            },
+        )
+
+
+class AttachmentNotFoundException(ResourceNotFoundException):
+    """Raised when an attachment is not found."""
+
+    def __init__(self, attachment_id: int | str):
+        super().__init__(
+            resource_type="Attachment",
+            identifier=str(attachment_id),
+            message=f"Attachment '{attachment_id}' not found",
+            error_code="ATTACHMENT_NOT_FOUND",
+        )