refactor(box): clean up sandbox subsystem code quality and efficiency

- Fix O(n²) stderr trimming in runtime.py with running length tracker
  - Remove dead code: RESERVED_CONTAINER_PATHS, _subprocess_wait_task,
    unused config_hash computation, unused imports
  - Deduplicate connection callback in BoxRuntimeConnector, parse URL once
  - Use enum comparison instead of stringly-typed spec.network.value check
  - Replace manual _result_to_dict/_session_to_dict with model_dump()
  - Cache NativeToolLoader tool definition and sandbox system guidance
  - Extract _is_path_under() helper to eliminate duplicated path checks
  - Import SANDBOX_EXEC_TOOL_NAME from native.py instead of redefining
  - Add JSON startswith guard in logging_utils to skip futile json.loads
  - Fix ruff lint errors (F401 unused imports, F841 unused variables)

src/langbot/pkg/pipeline/process/logging_utils.py

@@ -25,24 +25,25 @@ def format_result_log(
             if content.startswith('err:'):
                 return f'tool error: {cut_str(content)}'
 
-            try:
-                payload = json.loads(content)
-            except json.JSONDecodeError:
-                return cut_str(result.readable_str())
+            if content.startswith('{'):
+                try:
+                    payload = json.loads(content)
+                except json.JSONDecodeError:
+                    return cut_str(result.readable_str())
 
-            if isinstance(payload, dict):
-                status = payload.get('status', 'unknown')
-                exit_code = payload.get('exit_code')
-                backend = payload.get('backend', '')
-                stdout = str(payload.get('stdout', '')).strip()
-                summary = f'tool result: status={status}'
-                if exit_code is not None:
-                    summary += f' exit_code={exit_code}'
-                if backend:
-                    summary += f' backend={backend}'
-                if stdout:
-                    summary += f' stdout={cut_str(stdout)}'
-                return summary
+                if isinstance(payload, dict):
+                    status = payload.get('status', 'unknown')
+                    exit_code = payload.get('exit_code')
+                    backend = payload.get('backend', '')
+                    stdout = str(payload.get('stdout', '')).strip()
+                    summary = f'tool result: status={status}'
+                    if exit_code is not None:
+                        summary += f' exit_code={exit_code}'
+                    if backend:
+                        summary += f' backend={backend}'
+                    if stdout:
+                        summary += f' stdout={cut_str(stdout)}'
+                    return summary
 
         return cut_str(result.readable_str())