fix: make delete_sandbox idempotent when data plane returns "sandbox not found"

[Copilot]

Control plane (list_sandboxes) and data plane (delete_sandbox) can diverge: a TERMINATED sandbox remains visible in list results, but the data plane has already removed it, causing delete_sandbox to raise ClientError('Failed to stop sandbox: sandbox not found') instead of succeeding silently.

Changes

• agentrun/sandbox/client.py — In both delete_sandbox and delete_sandbox_async, when the data plane returns a non-SUCCESS response with a message containing "sandbox not found", return a Sandbox with the given sandbox_id instead of raising. Existing HTTP-level 404 handling (raises ResourceNotExistError) is unchanged.

• tests/unittests/sandbox/test_client.py — Added test_delete_sandbox_not_found_in_response_is_idempotent and its async counterpart covering the new code path.

Before / After

# Before — raises on already-gone TERMINATED sandbox
client.delete_sandbox("91d970881b8d4dcaa5636df64577545b--18ee62652b")
# ClientError: Failed to stop sandbox: sandbox not found

# After — idempotent success
result = client.delete_sandbox("91d970881b8d4dcaa5636df64577545b--18ee62652b")
result.sandbox_id  # "91d970881b8d4dcaa5636df64577545b--18ee62652b"