Fix: code supports matplotlib (#13724)

### What problem does this PR solve?

Code as "final" node: 

![img_v3_02vs_aece4caf-8403-4939-9e68-9845a22c2cfg](https://github.com/user-attachments/assets/9d87b8df-da6b-401c-bf6d-8b807fe92c22)

Code as "mid" node:

![img_v3_02vv_f74f331f-d755-44ab-a18c-96fff8cbd34g](https://github.com/user-attachments/assets/c94ef3f9-2a6c-47cb-9d2b-19703d2752e4)


### Type of change

- [x] New Feature (non-breaking change which adds functionality)

agent/sandbox/executor_manager/models/schemas.py

@@ -21,6 +21,13 @@ from pydantic import BaseModel, Field, field_validator
 from models.enums import ResourceLimitType, ResultStatus, RuntimeErrorType, SupportLanguage, UnauthorizedAccessType
 
 
+class ArtifactItem(BaseModel):
+    name: str
+    mime_type: str
+    size: int
+    content_b64: str
+
+
 class CodeExecutionResult(BaseModel):
     status: ResultStatus
     stdout: str
@@ -37,6 +44,9 @@ class CodeExecutionResult(BaseModel):
     unauthorized_access_type: Optional[UnauthorizedAccessType] = None
     runtime_error_type: Optional[RuntimeErrorType] = None
 
+    # File artifacts produced by code execution (images, PDFs, CSVs, etc.)
+    artifacts: list[ArtifactItem] = []
+
 
 class CodeExecutionRequest(BaseModel):
     code_b64: str = Field(..., description="Base64 encoded code string")