Resolve Heisenbug in StringHashTable._unique
When processing an invalid Unicode string, the exception handler for UnicodeEncodeError called `get_c_string` with an ephemeral repr value that could be garbage-collected the next time an exception was raised.  Issue #45929 demonstrates the problem.

This commit fixes the problem by retaining a Python reference to the repr value that underlies the C string until after all `values` are processed.

Wisdom from StackOverflow suggests that there's very small performance difference between pre-allocating the array vs. append if indeed we do need to fill it all the way, but because we only need references on exceptions, we expect that in the usual case we will append very few elements, making it faster than pre-allocation.

- [x] closes #45929
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Given the new Cython 3.0 changes about to be merged, are there any C-API folks who might know how to take this across the finish line? Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.