fix: harden OpenResponses URL input fetching

docs/gateway/openresponses-http-api.md

@@ -186,7 +186,11 @@ URL fetch defaults:
 
 - `files.allowUrl`: `true`
 - `images.allowUrl`: `true`
+- `maxUrlParts`: `8` (total URL-based `input_file` + `input_image` parts per request)
 - Requests are guarded (DNS resolution, private IP blocking, redirect caps, timeouts).
+- Optional hostname allowlists are supported per input type (`files.urlAllowlist`, `images.urlAllowlist`).
+  - Exact host: `"cdn.example.com"`
+  - Wildcard subdomains: `"*.assets.example.com"` (does not match apex)
 
 ## File + image limits (config)
 
@@ -200,8 +204,10 @@ Defaults can be tuned under `gateway.http.endpoints.responses`:
         responses: {
           enabled: true,
           maxBodyBytes: 20000000,
+          maxUrlParts: 8,
           files: {
             allowUrl: true,
+            urlAllowlist: ["cdn.example.com", "*.assets.example.com"],
             allowedMimes: [
               "text/plain",
               "text/markdown",
@@ -222,6 +228,7 @@ Defaults can be tuned under `gateway.http.endpoints.responses`:
           },
           images: {
             allowUrl: true,
+            urlAllowlist: ["images.example.com"],
             allowedMimes: ["image/jpeg", "image/png", "image/gif", "image/webp"],
             maxBytes: 10485760,
             maxRedirects: 3,
@@ -237,6 +244,7 @@ Defaults can be tuned under `gateway.http.endpoints.responses`:
 Defaults when omitted:
 
 - `maxBodyBytes`: 20MB
+- `maxUrlParts`: 8
 - `files.maxBytes`: 5MB
 - `files.maxChars`: 200k
 - `files.maxRedirects`: 3
@@ -248,6 +256,13 @@ Defaults when omitted:
 - `images.maxRedirects`: 3
 - `images.timeoutMs`: 10s
 
+Security note:
+
+- URL allowlists are enforced before fetch and on redirect hops.
+- Allowlisting a hostname does not bypass private/internal IP blocking.
+- For internet-exposed gateways, apply network egress controls in addition to app-level guards.
+  See [Security](/gateway/security).
+
 ## Streaming (SSE)
 
 Set `stream: true` to receive Server-Sent Events (SSE):