refactor(box): remove legacy in-process runtime code and clean up smells

After the architecture settled on always using an independent Box Runtime
service, several pieces of compatibility code and design shortcuts were
left behind. This commit cleans them up:

- Remove `LocalBoxRuntimeClient` and `create_box_runtime_client` from
  production code (moved to test-only helper).
- Remove unused `_clip_bytes` method from backend.
- Remove `__langbot_session_placeholder__` hack by making `BoxSpec.cmd`
  default to empty and validating non-empty only in `runtime.execute()`.
- Extract `get_box_config()` helper to eliminate 5× duplicated config
  access boilerplate.
- Remove `session_id`/`host_path`/`host_path_mode` from the LLM-facing
  tool schema to enforce request-scoped session isolation.
- Fix dual shutdown path: `NativeToolLoader.shutdown()` no longer calls
  `box_service.shutdown()` (handled by `Application.dispose()`).
- Simplify `_assert_session_compatible` with a loop.
- Inline client creation in `BoxRuntimeConnector`.
- Remove redundant `BOX__RUNTIME_URL` env var from docker-compose
  (auto-detected by code).

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

src/langbot/pkg/box/runtime.py

@@ -6,7 +6,7 @@ import datetime as dt
 import logging
 
 from .backend import BaseSandboxBackend, DockerBackend, PodmanBackend
-from .errors import BoxBackendUnavailableError, BoxSessionConflictError, BoxSessionNotFoundError
+from .errors import BoxBackendUnavailableError, BoxSessionConflictError, BoxSessionNotFoundError, BoxValidationError
 from .models import BoxExecutionResult, BoxExecutionStatus, BoxSessionInfo, BoxSpec
 
 _UTC = dt.timezone.utc
@@ -36,6 +36,8 @@ class BoxRuntime:
         self._backend = await self._select_backend()
 
     async def execute(self, spec: BoxSpec) -> BoxExecutionResult:
+        if not spec.cmd:
+            raise BoxValidationError('cmd must not be empty')
         session = await self._get_or_create_session(spec)
 
         async with session.lock:
@@ -183,38 +185,18 @@ class BoxRuntime:
             self.logger.warning(f'Failed to clean up box session {session_id}: {exc}')
 
     def _assert_session_compatible(self, session: BoxSessionInfo, spec: BoxSpec):
-        if session.network != spec.network:
-            raise BoxSessionConflictError(
-                f'sandbox_exec session {spec.session_id} already exists with network={session.network.value}'
-            )
-        if session.image != spec.image:
-            raise BoxSessionConflictError(
-                f'sandbox_exec session {spec.session_id} already exists with image={session.image}'
-            )
-        if session.host_path != spec.host_path:
-            raise BoxSessionConflictError(
-                f'sandbox_exec session {spec.session_id} already exists with host_path={session.host_path}'
-            )
-        if session.host_path_mode != spec.host_path_mode:
-            raise BoxSessionConflictError(
-                f'sandbox_exec session {spec.session_id} already exists with host_path_mode={session.host_path_mode.value}'
-            )
-        if session.cpus != spec.cpus:
-            raise BoxSessionConflictError(
-                f'sandbox_exec session {spec.session_id} already exists with cpus={session.cpus}'
-            )
-        if session.memory_mb != spec.memory_mb:
-            raise BoxSessionConflictError(
-                f'sandbox_exec session {spec.session_id} already exists with memory_mb={session.memory_mb}'
-            )
-        if session.pids_limit != spec.pids_limit:
-            raise BoxSessionConflictError(
-                f'sandbox_exec session {spec.session_id} already exists with pids_limit={session.pids_limit}'
-            )
-        if session.read_only_rootfs != spec.read_only_rootfs:
-            raise BoxSessionConflictError(
-                f'sandbox_exec session {spec.session_id} already exists with read_only_rootfs={session.read_only_rootfs}'
-            )
+        _COMPAT_FIELDS = (
+            'network', 'image', 'host_path', 'host_path_mode',
+            'cpus', 'memory_mb', 'pids_limit', 'read_only_rootfs',
+        )
+        for field in _COMPAT_FIELDS:
+            session_val = getattr(session, field)
+            spec_val = getattr(spec, field)
+            if session_val != spec_val:
+                display = session_val.value if hasattr(session_val, 'value') else session_val
+                raise BoxSessionConflictError(
+                    f'sandbox_exec session {spec.session_id} already exists with {field}={display}'
+                )
 
     @staticmethod
     def _session_to_dict(info: BoxSessionInfo) -> dict: