fix: restore os.Args after plugin completion and fix error return

[luojiyin1987]

Description

This PR fixes two issues in the plugin manager:

1. Fix error return value: The plugin stub was returning the wrong error variable when flag parsing failed
2. Restore os.Args after plugin completion: The global os.Args was being modified during plugin completion but not restored, which could cause side effects in subsequent code

Files changed:

• cli-plugins/manager/cobra.go:

  • Line 50: Return perr instead of err
  • Lines 63-66: Save and restore original os.Args using defer

• cli-plugins/manager/cobra_test.go:

  • Add tests for the above fixes