fix(contacts): tolerate non-string body in /api/contacts/import (#3638)

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.

routes/contacts_routes.py

@@ -729,8 +729,11 @@ def setup_contacts_routes():
     @router.post("/import")
     async def import_vcf(data: dict, _admin: str = Depends(require_admin)):
         """Import contacts from .vcf or CSV. Body: {"vcf": "..."} or {"csv": "..."}."""
-        text = data.get("vcf") or data.get("text") or ""
-        csv_text = data.get("csv") or ""
+        # Coerce defensively: a non-string vcf/text/csv (e.g. a number or list
+        # in the JSON body) would otherwise reach .strip() and 500 with an
+        # AttributeError instead of degrading to a clean "no data" response.
+        text = str(data.get("vcf") or data.get("text") or "")
+        csv_text = str(data.get("csv") or "")
         if text.strip():
             if "BEGIN:VCARD" not in text.upper():
                 return {"success": False, "error": "No vCard data found"}