Edge Functions Test panel always shows generic error for non-platform, hiding the real failure reason

**Bug**: On self-hosted Supabase, the Edge Functions **Test** panel in Studio always shows the same generic error — `Edge function returned an error` — no matter what actually failed (thrown exception, timeout, gateway/DNS issue, malformed input, anything). This happens 100% of the time, not intermittently. **Root cause**: `apps/studio/pages/api/edge-functions/test.ts` extracts the error message by reading `errorBody?.error`: ```ts error: { message: errorBody?.error || 'Edge function returned an error' } ``` But self-hosted's Kong-fronted edge runtime returns errors shaped as `{"message": "..."}`, not `{"error": "..."}`. The key never matches, so it always falls through to the hardcoded string. This logic was written for the platform-only version of this endpoint and never re-validated against self-hosted's actual response shape when self-hosted test support was added in #42350. **To reproduce** 1. Self-host Supabase (`supabase start`), deploy a function via `supabase functions deploy <name>`. 2. Studio → Edge Functions → the function → **Test** → send a request that fails. 3. Response shows the generic message regardless of the real cause. **Proof** — curl against the raw edge runtime vs. Studio's test-proxy endpoint, before/after the fix: ``` # the real error, straight from the edge runtime $ curl -X POST http://127.0.0.1:54321/functions/v1/og-images -d '{}' {"message":"name resolution failed"} # Studio's proxy, same failure — before fix (master@d5fde192d5) $ curl -X POST http://localhost:8082/api/edge-functions/test -d '{"url":"...","method":"POST","body":"{}","headers":{}}' {"status":503,"error":{"message":"Edge function returned an error"}} ``` Also confirmed in the Studio UI itself (Test → Send Request): before the fix shows "Edge function returned an error"; this is unchanged for gateway-level failures (see Wider fix below for why), but is now fixed for worker-level failures since the worker itself emits the right key. **Fix**: #47836 — fixes the self-hosted …