fix(runtime): fix return handling for reactive atoms + forward runtime errors to frontend

This commit fixes a bug introduced in `_build_atom_function` that
caused value producing component calls like `n = slider(...)` to
return `None`. Previously, `ast.Call` nodes were wrapped in an
`ast.Expr`, which did not yield a value. This has been corrected by
returning an `ast.Return(call_expr)` directly, restoring expected
behavior.

Additionally, runtime errors (e.g. divide-by-zero) are now forwarded
to the frontend during reruns. The `ScriptRunner.rerun` method checks
for registered errors and sends them via an `errors:result` message,
making error handling reactive and visible to the user.

Author: BloggerBust

preswald/engine/runner.py

@@ -213,8 +213,15 @@ async def rerun(self, new_widget_states: dict[str, Any] | None = None):
             components = self._service.get_rendered_components()
             logger.info(f"[ScriptRunner] Rendered {len(components)} components (rerun)")
 
-            if components:
-                await self.send_message({"type": "components", "components": components})
+            errors = self._service.get_errors(filename=self.script_path)
+            message_type = "errors:result" if len(errors) else "components"
+
+            if len(components) or len(errors):
+                await self.send_message({
+                    "type": message_type,
+                    "errors": errors or [],
+                    "components": components or []
+                })
                 if logger.isEnabledFor(logging.DEBUG):
                     logger.debug(f"[ScriptRunner] Sent components to frontend {components=}")
                 else:

preswald/engine/transformers/reactive_runtime.py

@@ -2478,13 +2478,26 @@ def _build_atom_function(
         # Create function parameters: (param0, param1, ...)
         args_ast = self._make_param_args(callsite_deps)
 
+        callsite_metadata = self._build_callsite_metadata(callsite_node, self.filename)
+        decorator = self._create_workflow_atom_decorator(
+            atom_name,
+            callsite_deps,
+            callsite_metadata=callsite_metadata
+        )
+
         # Normalize call_expr into a body list
         if isinstance(call_expr, list):
             body = call_expr
         elif isinstance(call_expr, ast.Assign) or isinstance(call_expr, ast.Expr):
             body = [call_expr]
         elif isinstance(call_expr, ast.Call):
-            body = [ast.Expr(value=call_expr)]
+            return_stmt = ast.Return(value=call_expr)
+            return ast.FunctionDef(
+                name=atom_name,
+                args=args_ast,
+                body=[return_stmt],
+                decorator_list=[decorator],
+            )
         else:
             self._safe_register_error(
                 node=call_expr,
@@ -2494,13 +2507,6 @@ def _build_atom_function(
             )
             return None
 
-        callsite_metadata = self._build_callsite_metadata(callsite_node, self.filename)
-        decorator = self._create_workflow_atom_decorator(
-            atom_name,
-            callsite_deps,
-            callsite_metadata=callsite_metadata
-        )
-
         # Append appropriate return statement
         if isinstance(return_target, str):
             body.append(ast.Return(value=ast.Name(id=return_target, ctx=ast.Load())))