`setAllowedHosts`: blocked outbound requests return opaque `520 "Origin is disallowed"` — allow customizing the block response

**Summary**

When `setAllowedHosts()` is used, any outbound request to a non-allowlisted host is rejected with `520` and the fixed plain-text body `Origin is disallowed` (from `@cloudflare/containers`). Sandboxed code that calls `.json()` on the response — the common case — crashes with an opaque parse error, and there's no supported way to customize the block response without giving up the allowlist gate entirely.

Version: `@cloudflare/sandbox@0.12.1`.

**Repro**

```python
import requests
resp = requests.post("https://disallowed-host.example", json={})
print(resp.json())
```

- **Node/JS** runtime: `Unexpected token 'O', "Origin is disallowed" is not valid JSON`
- **Python** runtime: `json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)` — body not echoed, indistinguishable from an empty response.

Both runtimes receive the same `520 "Origin is disallowed"`; the difference is only how each language's JSON parser formats the decode error.

**Why it's hard to handle today**

In `@cloudflare/containers`, the `allowedHosts` gate runs *before* any `outbound` / `outboundByHost` handler and hardcodes `new Response('Origin is disallowed', { status: 520 })`. So the only way to return a custom (e.g. structured-JSON) block response is to drop `setAllowedHosts` and re-implement allow/deny inside a catch-all `outbound` handler — which moves the egress security gate into user code.

**Request**

Provide a supported way to customize the blocked-host response while keeping the `allowedHosts` gate — e.g. an optional `onDisallowedHost(request) => Response` hook, or a configurable default block response. Returning structured JSON (e.g. `{ "error": "host_not_allowed", "host": "..." }`) would let calling code surface a clear, parseable error in every runtime.

Start by inspecting the @cloudflare/containers allowedHosts gate and how it precedes outbound and outboundByHost handlers. Reproduce the blocked request in the Node/JS and Python examples, then define and test a supported customization that preserves the allowlist gate and returns a parseable block response.

Written by the indexing model from the issue text.