fix: handle subscription plan errors without error_description field

[terwey]

Summary

Fixed a nil pointer dereference panic that occurred when the API returns error responses without an error_description field, particularly for subscription plan limitation errors.

Problem

When the 3Commas API returns a 429 error for subscription plan limitations, the response only contains the error field without the optional error_description field:

{
  "error": "The request type 'read' is not available with your current subscription plan. Please upgrade your plan to use this type of request."
}

The SDK's error handling code always attempted to dereference ErrorDescription, causing a panic:

runtime error: invalid memory address or nil pointer dereference

Changes

SDK Fix

• Updated APIError.Error() method to check if ErrorDescription is nil before dereferencing
• Falls back to the Error field when ErrorDescription is not present
• Ensures robust error handling for all API error responses

OpenAPI Spec Enhancement

• Updated RateLimitExceeded response description to clarify it covers both rate limiting and subscription plan limitations
• Added examples for both traditional rate limit errors and subscription plan limitation errors

Testing

• Added test case TestListBots/Subscription_not_active to verify the fix
• Test validates proper error message formatting without panic

Impact

• Bug Fix: Eliminates panic when handling subscription plan limitation errors
• Better Error Messages: Users now see clear error messages for subscription issues
• Improved Documentation: OpenAPI spec better reflects actual API behavior