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

[thaJeztah]

whoops! I started reviewing, but didn't submit 🙈

[thaJeztah]

Nice catch; while updating; perhaps just inline it;

if err := flags.Parse(args); err != nil {
	return err

[thaJeztah]

Looks like my IDE (and possibly linters) has trouble detecting that the returned command will never be nil (due to the asserts); it's probably OK to just return the errors instead

[thaJeztah]

I know we use the fs package still in some other places in this package, but I'm .. slightly trying to get rid of it (in most cases it's overkill); this one can probably be simplified using stdlib;

Also looks like we don't use the metadata anywhere for assertions, so we could just make this all "static" (until there's a specific reason to customise it);

	tmpDir := t.TempDir()
	const cliPlugin = `
#!/bin/sh
printf '%s' '{"SchemaVersion":"0.1.0","Vendor":"e2e-testing"}'
`
	err := os.WriteFile(filepath.Join(tmpDir, "docker-testplugin"), []byte(cliPlugin), 0o777)

[thaJeztah]

Probably fine here to simplify and just patch the config;

cli.ConfigFile().CLIPluginsExtraDirs = []string{tmpDir}