fix(harness): reset local sandbox singleton with provider lifecycle (#2834)

* Fix local sandbox singleton reset on provider lifecycle

* Fix local sandbox singleton reset on provider reset

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/sandbox/local/local_sandbox_provider.py

@@ -119,3 +119,13 @@ class LocalSandboxProvider(SandboxProvider):
         # For Docker-based providers (e.g., AioSandboxProvider), cleanup
         # happens at application shutdown via the shutdown() method.
         pass
+
+    def reset(self) -> None:
+        # reset_sandbox_provider() must also clear the module singleton.
+        global _singleton
+        _singleton = None
+
+    def shutdown(self) -> None:
+        # LocalSandboxProvider has no extra resources beyond the shared
+        # singleton, so shutdown uses the same cleanup path as reset.
+        self.reset()

backend/packages/harness/deerflow/sandbox/sandbox_provider.py

@@ -37,6 +37,10 @@ class SandboxProvider(ABC):
         """
         pass
 
+    def reset(self) -> None:
+        """Clear cached state that survives provider instance replacement."""
+        pass
+
 
 _default_sandbox_provider: SandboxProvider | None = None
 
@@ -65,11 +69,18 @@ def reset_sandbox_provider() -> None:
     The next call to `get_sandbox_provider()` will create a new instance.
     Useful for testing or when switching configurations.
 
+    Providers can override `reset()` to clear any module-level state they keep
+    alive across instances (for example, `LocalSandboxProvider`'s cached
+    `LocalSandbox` singleton). Without it, config/mount changes would not take
+    effect on the next acquire().
+
     Note: If the provider has active sandboxes, they will be orphaned.
     Use `shutdown_sandbox_provider()` for proper cleanup.
     """
     global _default_sandbox_provider
-    _default_sandbox_provider = None
+    if _default_sandbox_provider is not None:
+        _default_sandbox_provider.reset()
+        _default_sandbox_provider = None
 
 
 def shutdown_sandbox_provider() -> None: