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

[luojiyin1987]

- What I did

Fix 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

- How I did it

1. Changed the error return from err to perr when flag parsing fails
2. Added code to save the original os.Args before modification and use defer to restore it
3. Added comprehensive tests to verify both fixes

- Files changed:

File	Change
cli-plugins/manager/cobra.go	Fix error return + restore os.Args
cli-plugins/manager/cobra_test.go	Add regression tests

- How to verify it

The new tests verify both scenarios.

- Description for the changelog

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

Fixes #6816