Return raw dict from sandbox, let harness handle truncation

Matches the updated SandboxFunc contract — sandbox returns a dict,
execute_tool applies truncation and serializes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sandbox_e2b.py

@@ -2,12 +2,11 @@
 
 from __future__ import annotations
 
-import json
 from pathlib import Path
 
 from e2b_code_interpreter import Sandbox
 
-from llm_harness.sandbox import TIMEOUT_SECONDS, _truncate
+from llm_harness.sandbox import TIMEOUT_SECONDS
 
 # Reuse a sandbox across tool calls within a session.
 # The caller manages the lifecycle via create/close.
@@ -59,7 +58,7 @@ def run_python(
     workspace: Path | None = None,
     scratch_dir: Path | None = None,
     timeout: int = TIMEOUT_SECONDS,
-) -> str:
+) -> dict:
     """Execute Python code in an E2B sandbox. Same interface as sandbox.run_python."""
     sandbox = get_or_create_sandbox(workspace, scratch_dir)
 
@@ -81,20 +80,16 @@ def run_python(
         else:
             exit_code = 0
 
-        return json.dumps(
-            {
-                "stdout": _truncate(stdout),
-                "stderr": _truncate(stderr),
-                "exit_code": exit_code,
-                "timed_out": False,
-            }
-        )
+        return {
+            "stdout": stdout,
+            "stderr": stderr,
+            "exit_code": exit_code,
+            "timed_out": False,
+        }
     except TimeoutError:
-        return json.dumps(
-            {
-                "stdout": "",
-                "stderr": "Execution timed out.",
-                "exit_code": -1,
-                "timed_out": True,
-            }
-        )
+        return {
+            "stdout": "",
+            "stderr": "Execution timed out.",
+            "exit_code": -1,
+            "timed_out": True,
+        }