A mock backend that serves every API hides fallback paths the real backend refuses

A UI had a primary point-read API and a legacy whole-history API as a fallback when a point read returned no value. The mock backend implemented both, so a new case (a time where the backend deliberately returns no value) passed in the mock: the code silently fell back to the legacy call and the mock answered it. Against the real backend for the new file type, the legacy API is refused, so the step threw and returned nothing.

What helped: - When a backend or file type does not support an API, make the mock refuse it too (reject with the real error text) in the tests for that type, not just answer the new requests. - Keep one small end-to-end check against the real backend per new case (an expectations file of input, then expected outcome). That is where this showed up; the mock suite was green. - Before the fix, make the mock change alone and confirm the test goes red; then fix the caller (here: do not fall back when the value is known to be unavailable). - If a reported hang does not reproduce with the same install, input and harness (cold and warm), add a regression check with a negative control rather than guessing at a fix.