fix: use default values for optional workflow input variables (#28546) (#28527)

Co-authored-by: Claude <noreply@anthropic.com>

api/core/app/app_config/entities.py

@@ -112,6 +112,7 @@ class VariableEntity(BaseModel):
     type: VariableEntityType
     required: bool = False
     hide: bool = False
+    default: Any = None
     max_length: int | None = None
     options: Sequence[str] = Field(default_factory=list)
     allowed_file_types: Sequence[FileType] | None = Field(default_factory=list)