[lyelgate]

📝 Description

What does this PR do and why is this change necessary?

Fixes #868.

help now works like the --help flag when given as the command or the action:

Command	Before	After
linode-cli help	Unrecognized command help (exit 1)	same as linode-cli --help
linode-cli help linodes	Unrecognized command help (exit 1)	same as linode-cli linodes --help
linode-cli linodes help	Action not found for command linodes: help (exit 2)	same as linode-cli linodes --help
linode-cli help linodes create	Unrecognized command help (exit 1)	same as linode-cli linodes create --help
linode-cli obj help	(handled by the plugin)	same as linode-cli obj --help

This is a new take on #878, which was closed with review feedback outstanding. The feedback is addressed:

• sys.argv is changed in place, not reassigned. linodecli/__init__.py holds from sys import argv and builds plugin arguments from it, so reassigning would desync them. help_subcommand_to_flag deletes the help token and appends --help on the same list, then main() re-parses with the same parser.
• Only the command and action positions are rewritten. They come from the parsed arguments, so global options before the command (linode-cli --json linodes help) still work, and help as a value is untouched: linode-cli obj mb help still creates a bucket named help, and linode-cli linodes view help still passes help as the ID.
• The tests are plain unit tests with no subprocess and no hard-coded paths, so they run on Windows too. They cover both the help <command> and <command> help forms.

✔️ How to Test

What are the steps to reproduce the issue or verify the changes?

linode-cli help linodes
linode-cli linodes help
linode-cli help linodes create
linode-cli obj help

With a baked spec, I compared the output of each of these and of linode-cli help with its --help equivalent. They are identical, with exit code 0.

How do I run the relevant unit/integration tests?

LINODE_CLI_TEST_MODE=1 pytest tests/unit/test_arg_helpers.py -k help_subcommand

The tests run each case through the real register_args parser and check the rewritten arguments: six rewritten forms (including a global option before the command, a plugin, and that the list is changed in place) and four that must be left alone. black, isort, autoflake and pylint pass, and so does make test-unit (250 tests) with a baked spec.