🐛 Respect ZDOTDIR for zsh completion install

[lawrence3699]

Fixes #171

When ZDOTDIR is set, --install-completion zsh was still writing the init line to ~/.zshrc, so the active zsh config never picked it up.

This uses ZDOTDIR/.zshrc when ZDOTDIR resolves to an absolute path and keeps the existing fallback to ~/.zshrc.

Validation:

• _TYPER_RUN_INSTALL_COMPLETION_TESTS=1 pytest -q tests/test_completion/test_completion_install.py -k 'completion_install_zsh'
• pytest -q tests/test_completion/test_completion.py -k 'completion_source_zsh'

[Copilot]

Pull request overview

This PR fixes zsh completion installation so that when ZDOTDIR is set, Typer appends the initialization line to the active zsh config ($ZDOTDIR/.zshrc) instead of always writing to ~/.zshrc, addressing issue #171.

Changes:

• Add _get_zshrc_path() to choose $ZDOTDIR/.zshrc when ZDOTDIR expands to an absolute path, otherwise fall back to ~/.zshrc.
• Update install_zsh() to use the new zshrc path resolver.
• Add a regression test validating --install-completion zsh respects ZDOTDIR.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
typer/_completion_shared.py	Resolve the zsh rc file path via ZDOTDIR (absolute-path cases) for zsh completion installation.
tests/test_completion/test_completion_install.py	Add coverage to ensure installation modifies $ZDOTDIR/.zshrc and not $HOME/.zshrc when ZDOTDIR is set.

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[svlandeg]

Duplicate of #1553, closing