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## File properties and search field fixes

#### Notes

* A **recognised** public field name that doesn't apply to the record type you're searching (for
  example `value` on an account, or `trades` on a vendor) now returns **`400`** naming the field,
  instead of an empty `200`. If you were relying on an empty result for one of these, you'll now get
  an explicit error — which is the point: an empty list was indistinguishable from "no matches".
* A field name the API does **not** recognise as a standard field is still treated as a possible
  custom property, unchanged. An empty result there still means "no records matched" — we can't tell
  a typo from a real custom property name without knowing your company's columns, so only fields we
  can prove don't apply are rejected.
* `trades` holds a list of values, so a scalar comparison matches if any entry equals your value.
  Range and substring operators are rejected with a `400` rather than silently matching nothing.