fix: accept parsed gift dates

[be-student]

Fixes #163.

Normalizes the gift-date column before scikit-learn validates the mixed-dtype frame, so both strings and datetime64 inputs work. Invalid non-missing dates now raise a column-specific error.

Validation: make ci (1,951 passed, 25 skipped; 97.44% coverage).

[shivamlalakiya]

@be-student three merged fixes in one day, and all three were real defects rather than coverage padding. #179 in particular closed a silent no-op: a wealth_cols list that matched no column was accepted and quietly ignored, so a user would have got numbers back with no indication that the transformer had done nothing. That's the failure mode I most want caught, because nothing downstream ever raises.

You clearly read the codebase rather than just the issue text, so I'd rather point you at something open-ended than another labelled issue.

#110 is the one unresolved design question in the hospital-fundraising path: index-encounter keying versus the PHI-free bundle contract, which tests/test_no_phi_in_bundles.py currently depends on. I've written up why it's deferred rather than decided, but a deferral I argued with myself about is worth less than one someone else has pushed back on. If you have a view, the thread is the right place for it.

Separately, if anything about getting set up here was annoying, I'd like to hear that too: #188