feat(shell): add background command execution with output redirection and timeout support

tests/unit/test_computer_shell_tool.py

@@ -0,0 +1,15 @@
+from astrbot.core.tools.computer_tools.shell import (
+    _redirect_background_stdout_command,
+)
+
+
+def test_background_shell_command_redirects_output_to_file():
+    command = _redirect_background_stdout_command(
+        "python -c 'print(123)'",
+        output_path="/tmp/astrbot shell output.log",
+        local_runtime=False,
+    )
+
+    assert command == (
+        "python -c 'print(123)' > \"/tmp/astrbot shell output.log\" 2>&1"
+    )