diffs: add configurable viewer base URL (#59341)

Merged via squash.

Prepared head SHA: 3c2a84849f
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

extensions/diffs/src/store.test.ts

@@ -340,6 +340,14 @@ describe("createDiffsHttpHandler", () => {
       allowRemoteViewer: false,
       expectedStatusCode: 404,
     },
+    {
+      name: "blocks proxied loopback requests when trusted proxies are configured",
+      request: localReq,
+      headers: { "x-forwarded-for": "203.0.113.10" },
+      trustedProxies: ["127.0.0.1"],
+      allowRemoteViewer: false,
+      expectedStatusCode: 404,
+    },
     {
       name: "allows remote access when allowRemoteViewer is enabled",
       request: remoteReq,