tavily_search, serper_search and google_pse_search parsed response.json()
inside the network try block, which only caught httpx.RequestError and
RateLimitError. When a provider returned a non-JSON body (an HTML error page, a
truncated/empty body, a gateway 5xx), response.json() raised an UNCAUGHT
json.JSONDecodeError that aborted the search in the background — exactly the
'search engines other than SearXNG fail in the background' symptom.
brave_search already handles this correctly: it parses JSON in its own try
block and returns [] on json.JSONDecodeError. Mirror that in the other three
providers so a malformed provider response degrades to no-results instead of
propagating an exception.
Adds tests/test_search_provider_json.py: a non-JSON 200 body now yields [] for
tavily, serper, google_pse, and brave (the last guards the reference behaviour).
Co-authored-by: NubsCarson <nubs@nubs.site>
