Fix zsh completion for commands with spaces

[lin-hongkuan]

Fixes #153.
Fixes #360.

Summary

• keep zsh completion command values unquoted for commands with spaces, so accepting a completion does not insert literal quote characters
• add a generated command-name list for zsh completion and match the longest command prefix before falling back to the previous first-word lookup
• update the zsh completion fixture and add a regression assertion for spaced command completions

Tests

• PYTHONPATH=src python -m pytest tests/commands/completion/test_completions_command.py -q
• python -m ruff check src/cleo/commands/completions_command.py src/cleo/commands/completions/templates.py tests/commands/completion/test_completions_command.py
• python -m ruff format --check src/cleo/commands/completions_command.py src/cleo/commands/completions/templates.py tests/commands/completion/test_completions_command.py
• python -m towncrier check --compare-with origin/main