Name the zsh completion function _<prog> so autoload works

[apoorva-01]

Fixes #1864.

Problem

zsh autoloads a completion from a file in $fpath named after the command (say _prog), and the autoload convention requires the function that file defines to match the filename. Typer generated the function as _<prog>_completion, so when it's loaded that way the completion silently fails. zsh autoloads _prog, finds no _prog function, and falls back to filename completion instead of command completion:

#compdef hf
_hf_completion() { ... }     # autoloaded file is named "_hf", so there's no "_hf" function
compdef _hf_completion hf

It hits two paths. Packaging (prog --show-completion=zsh > $fpath/_hf), which is what the issue reports. And --install-completion, which writes the script to ~/.zfunc/_<prog> and adds fpath+=~/.zfunc; autoload -Uz compinit, so the same autoload route.

Fix

Generate _<prog> instead of _<prog>_completion. The #compdef/compdef lines already register the completion, so the _completion suffix wasn't buying anything, and dropping it makes the function name match the autoloaded filename:

#compdef hf
_hf() { ... }
compdef _hf hf

The zsh script is rendered in two places, get_completion_script (the --show-completion/--install-completion path) and ZshComplete.source() (the _<PROG>_COMPLETE=source_zsh runtime path), so I updated both to keep them consistent. It's zsh-only. bash/fish/PowerShell output is unchanged (bash keeps _<prog>_completion, which has no filename constraint).

Tests

Updated the three existing zsh completion assertions (test_completion.py, test_completion_show.py, test_completion_install.py) to expect _<prog> and to check the old _completion suffix is gone. They pass with the fix and fail without it. Full tests/test_completion/ suite passes locally (77 passed). ruff check/format clean.

[phalberg]

Hey,
I am not maintainer of the project but nonetheless, some thoughts of this PR:

1. Please follow the guidelines, as per: https://tiangolo.com/open-source/contributing/#start-with-a-discussion, the issue/discussion has never gotten approval from a maintainer to open a PR for it (yet).
2. Please add a statement about AI usage (if any).

Thanks! I will not be replying to questions for this comment as I do not want to burden the maintainers time more.