Closes #2378 add test for `get_imputation_target_date()`

[zdz2101]

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[zdz2101]

@bms63 @millerg23 I was mistaken in that the internal functions weren't being loaded in the test suite, I probably had somethin wrong with my session, nothing needs to be done to address this, just regular ol' additional tests

[millerg23]

I assume failing checks are around ERROR message updates. Happy to Approve this, once checks running clean.

[bms63]

@zdz2101 @millerg23 is this one good to merge?

[zdz2101]

@zdz2101 @millerg23 is this one good to merge?

Should be once cicd goes through!