import_vcf built `text = data.get("vcf") or data.get("text") or ""`, so a
non-string JSON value (a number, list, etc.) stayed in place and the following
`text.strip()` raised AttributeError, returning HTTP 500. Coerce vcf/text/csv
with str() so non-string input degrades to the existing structured "no data"
response, matching the file's convention elsewhere.
Adds tests/test_contacts_import_nonstring.py covering non-string vcf, non-string
csv, and an empty body.
Mazen Tamer Salah