dev/core#5370 Fix bug handling soft credits in civi-import

[eileenmcnaughton]

partial port of #30807

[civibot]

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[civibot]

The issue associated with the Pull Request can be viewed at https://lab.civicrm.org/dev/core/-/issues/5370

[totten]

Most of the testing was on the original PR. Just as a sanity-check of the backport, I made sure that the UI was generally loading (and that an import-mapping was saved).

[totten]

Most tests pass. The last remaining test-suite would run for a while, but I'm gonna go ahead and merge. That test is pure-PHP, and this update is pure client-side JS, so there's no plausible path to true-negative.