APIv4: Normalize option list descriptions as plain text

[colemanw]

Overview

Ensure the description field comes through as plain text when fetching option lists from APIv4.

Before

After

Technical Details

Our schema is inconsistent about whether description fields allow html, but it's usually assumed to be plain text, so this PR uses strip_tags() to standardize it.

[civibot]

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[eileenmcnaughton]

This only affects searchkit which remains fast moving code. I'm testing intermittently rather than each PR

[colemanw]

Thanks @eileenmcnaughton.
This actually affects the whole of APIv4 but I feel like it's the right fix, as one of its purposes is to smooth out irregularities and quirks in our schema.

[eileenmcnaughton]

@colemanw argh you are right - ping @totten in case I missed any caveats on this