fix(security): release the vendor stream when metering rejects the request

The fail-closed branch returned 500 with the ElevenLabs response body
still open, so synthesis and download kept consuming vendor and runtime
resources for a caller that was already rejected. Cancel it before
returning, and assert the cancellation in the test.

Author: waleedlatif1

apps/sim/app/api/proxy/tts/stream/route.test.ts

@@ -82,20 +82,26 @@ function validBody(overrides: Record<string, unknown> = {}) {
   }
 }
 
-/** Minimal ElevenLabs stub returning a readable audio body. */
+/**
+ * Minimal ElevenLabs stub returning a readable audio body. Returns the `cancel`
+ * spy so tests can assert the vendor stream is released when we reject.
+ */
 function mockElevenLabsAudio() {
+  const stream = new ReadableStream({
+    start(controller) {
+      controller.enqueue(new Uint8Array([0x49, 0x44, 0x33]))
+      controller.close()
+    },
+  })
+  const cancel = vi.fn(() => stream.cancel())
   global.fetch = vi.fn().mockResolvedValue({
     ok: true,
     status: 200,
     statusText: 'OK',
-    body: new ReadableStream({
-      start(controller) {
-        controller.enqueue(new Uint8Array([0x49, 0x44, 0x33]))
-        controller.close()
-      },
-    }),
+    body: { getReader: () => stream.getReader(), cancel },
     // double-cast-allowed: minimal fetch stub for the ElevenLabs stream call
   }) as unknown as typeof fetch
+  return cancel
 }
 
 beforeEach(() => {
@@ -257,14 +263,16 @@ describe('POST /api/proxy/tts/stream — attribution', () => {
     expect(mockRecordUsage).not.toHaveBeenCalled()
   })
 
-  it('refuses to stream audio it could not record a charge for', async () => {
+  it('refuses to stream audio it could not record a charge for, releasing the vendor stream', async () => {
+    const cancel = mockElevenLabsAudio()
     queueTableRows(schemaMock.chat, [publicChatRow])
     mockRecordUsage.mockRejectedValue(new Error('ledger unavailable'))
 
     const res = await POST(createMockRequest('POST', validBody()))
 
     expect(res.status).toBe(500)
     expect(res.headers.get('Content-Type')).not.toBe('audio/mpeg')
+    expect(cancel).toHaveBeenCalledTimes(1)
   })
 
   it('rejects an unknown chat without touching the platform key', async () => {

apps/sim/app/api/proxy/tts/stream/route.ts

@@ -187,6 +187,8 @@ export const POST = withRouteHandler(async (request: NextRequest) => {
       })
     } catch (err) {
       logger.error('Failed to record voice output usage, refusing to stream:', err)
+      // Release the open vendor stream; nothing will drain it once we reject.
+      await response.body?.cancel().catch(() => {})
       return new Response('Unable to record usage for this request', { status: 500 })
     }