fix(admission): reserve Responses and Messages bodies before clone (#10814)

Merged — validated together with a batch of related RaviTharuma PRs in one combined worktree (typecheck:core clean, complexity/file-size/changelog gates green, focused tests passing). Thanks for the contribution!

src/shared/middleware/withChatAdmission.ts

@@ -0,0 +1,48 @@
+/**
+ * Compose process-wide chat admission in front of a route handler.
+ *
+ * Uses the shipped `admitChatRequest` budget/fairness controller — it does not
+ * introduce a second admission path. Call this *outside* `withInjectionGuard`
+ * so a large `/v1/responses` or `/v1/messages` body is reserved (or 503-shed)
+ * before `request.clone()` / `.json()`.
+ */
+import {
+  admitChatRequest,
+  CHAT_ADMISSION_QUEUE_MAX_MS,
+  releaseChatAdmissionAfterHandler,
+  resolveSessionId,
+  type ChatAdmissionController,
+} from "./chatBodyAdmission";
+
+type RouteHandler = (request: Request, ...args: any[]) => Promise<Response> | Response;
+
+export function withChatAdmission(
+  handler: RouteHandler,
+  options: {
+    controller?: ChatAdmissionController;
+    queueMs?: number;
+    largeBodyBytes?: number;
+    hardMaxBytes?: number;
+  } = {}
+): RouteHandler {
+  return async function admittedHandler(request: Request, ...args: any[]) {
+    const sessionId = resolveSessionId(request);
+    const admission = await admitChatRequest(request, {
+      sessionId,
+      queueMs: options.queueMs ?? CHAT_ADMISSION_QUEUE_MAX_MS,
+      controller: options.controller,
+      largeBodyBytes: options.largeBodyBytes,
+      hardMaxBytes: options.hardMaxBytes,
+    });
+    if (admission.admit === false) return admission.response;
+    try {
+      return await releaseChatAdmissionAfterHandler(
+        Promise.resolve(handler(admission.request, ...args)),
+        admission.lease
+      );
+    } catch (error) {
+      admission.lease?.release();
+      throw error;
+    }
+  };
+}