Fix Fish completion for Fire(fn)

[reginaldalfret]

Summary

This PR fixes Fish shell completion when fire.Fire is called on a top-level function (fire.Fire(fn)), analogously to PR #336 which previously resolved this behavior for Bash completion.

Background & Root Cause

In fire/completion.py, _FishScript iterates through commands_set constructed from subcommands_map.keys() and options_map.keys(). When fire.Fire targets a top-level function, subcommands_map and options_map are empty dictionaries, so commands_set remained empty. As a result, no complete -c <name> ... directives were emitted for the function's arguments.

Solution

Add name to commands_set in _FishScript (identical to the fix applied to _BashScript in #336):

commands_set = set()
commands_set.add(name)
commands_set = commands_set.union(set(subcommands_map.keys()))
commands_set = commands_set.union(set(options_map.keys()))

Testing

• Added an assertion in TabCompletionTest.testFnFishScript to verify that complete -c identity -n '__fish_using_command identity; is emitted in the generated fish script.
• Verified unit tests pass with pytest.
• Verified style and lint checks with pylint (10.00/10) and ty.

Fixes #418