fix(list): show help when no subcommand is provided

[abicky]

Description

Running az-pim-cli list without a subcommand currently produces no output, which can be confusing for users because it is not obvious that a subcommand is required.

This PR changes the behavior to display the command help when no subcommand is specified, making the command more user-friendly and improving discoverability of the available subcommands.

Before:

$ go run . list

After:

$ go run . list
Query Azure PIM for eligible role assignments

Usage:
  az-pim-cli list [flags]
  az-pim-cli list [command]

Aliases:
  list, l, ls

Available Commands:
  group       Query Azure PIM for eligible group assignments
  resource    Query Azure PIM for eligible resource assignments (azure resources)
  role        Query Azure PIM for eligible Entra role assignments

Flags:
  -h, --help   help for list

Global Flags:
      --cloud string    Which Azure environment to use ('global', 'usgov', 'china') (default "global")
  -c, --config string   config file (default is $HOME/.az-pim-cli.yaml)
      --debug           Enable debug logging

Use "az-pim-cli list [command] --help" for more information about a command.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contribution guidelines
• My changes follow the Styleguide of this project
• I have run the pre-commit hooks included in this repository
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• All GitHub Actions workflows for this branch/PR have run successfully

[netr0m]

Thanks for your contribution, @abicky! Approved - will include as part of the next release.