Fix zsh parse error in pipenv shell command

[matteius]

Summary

Fixes #6503

#6462 introduced a regression where pipenv shell fails on ZSH with a parse error:

eval "_pipenv_old_deactivate() { $(declare -f deactivate | tail -n +2) }";
zsh: parse error near `}'

Root Cause

The previous implementation used declare -f inside an eval statement for both bash and zsh. However, zsh handles command substitution differently - it expands $() before parsing the function definition, causing a parse error.

Solution

This fix separates the zsh and bash cases:

• zsh: Uses functions -c to copy function definitions (same approach as fish)
• bash: Continues using declare -f which works correctly in bash

Testing

Enhanced the existing unit test to verify zsh uses functions -c (not declare -f) to prevent future regressions.

Test Results

tests/unit/test_core.py::test_deactivate_wrapper_script_includes_unset_pipenv_active PASSED

---

Pull Request opened by Augment Code with guidance from the PR author