docs: fix HTTP gateway streaming callback return type (#296)

## Summary

The streaming callback in the HTTP gateway protocol spec must return a
non-opt `StreamingCallbackHttpResponse`. The spec incorrectly wrapped
the return type in `opt`, which no real gateway accepts — candid cannot
coerce wire-type `opt T` into the expected `T`, so serving any
multi-chunk (streamed) asset fails to decode.

Fixed all three occurrences:
- `public/references/http-gateway.did` (downloadable interface)
- `docs/references/http-gateway-protocol-spec.md` — full Canister HTTP
Interface block
- `docs/references/http-gateway-protocol-spec.md` — Response Body
Streaming Interface block

```diff
- callback: func (StreamingToken) -> (opt StreamingCallbackHttpResponse) query;
+ callback: func (StreamingToken) -> (StreamingCallbackHttpResponse) query;
```

Note: only the callback's **return type** changes. The `token : opt
Token` field inside `StreamingCallbackHttpResponse` stays optional — a
null *token* (not a null response) terminates the stream.

## Verification

The real gateway decodes the callback reply into a non-opt
`StreamingCallbackHttpResponse`:

`ic-gateway` → `ic-http-gateway-protocol` (tag `v0.5.1`) → `ic-agent` /
`ic-utils`

- [`ic-http-gateway-protocol` @ v0.5.1 —
`response_handler.rs#L116-L120`](https://github.com/dfinity/ic-http-gateway-protocol/blob/v0.5.1/packages/ic-http-gateway-protocol/src/response/response_handler.rs#L116-L120):
calls `http_request_stream_callback` and matches
`Ok((StreamingCallbackHttpResponse { body, token },))` — the bare
struct, no `Option`.
- [`ic-agent` —
`ic-utils/src/interfaces/http_request.rs`](https://github.com/dfinity/agent-rs/blob/main/ic-utils/src/interfaces/http_request.rs):
`http_request_stream_callback -> SyncCall<Value =
(StreamingCallbackHttpResponse,)>`, and the callback CandidType
`func((ArgToken) -> (StreamingCallbackHttpResponse) query)`.

Detected while adding e2e streaming tests for certified-assets
(dfinity/certified-assets#96). `npm run build` passes.

Author: lwshang

docs/references/http-gateway-protocol-spec.md

@@ -423,7 +423,7 @@ type StreamingCallbackHttpResponse = record {
 
 type StreamingStrategy = variant {
     Callback: record {
-        callback: func (StreamingToken) -> (opt StreamingCallbackHttpResponse) query;
+        callback: func (StreamingToken) -> (StreamingCallbackHttpResponse) query;
         token: StreamingToken;
     };
 };
@@ -509,7 +509,7 @@ type StreamingCallbackHttpResponse = record {
 
 type StreamingStrategy = variant {
     Callback: record {
-        callback: func (StreamingToken) -> (opt StreamingCallbackHttpResponse) query;
+        callback: func (StreamingToken) -> (StreamingCallbackHttpResponse) query;
         token: StreamingToken;
     };
 };

public/references/http-gateway.did

@@ -38,7 +38,7 @@ type StreamingCallbackHttpResponse = record {
 
 type StreamingStrategy = variant {
   Callback: record {
-    callback: func (StreamingToken) -> (opt StreamingCallbackHttpResponse) query;
+    callback: func (StreamingToken) -> (StreamingCallbackHttpResponse) query;
     token: StreamingToken;
   };
 };