fix(agent-proxy): cancel upstream reader on abort so segment timer can actually interrupt parseSse

Root cause of "segment_end never written, response just runs until
Netlify kills it": the Fetch spec only honors AbortSignal for the
request/header phase. Once you have `res.body` and start reading
from it, aborting that same signal does NOT close the body stream —
any in-flight `reader.read()` hangs indefinitely.

So when the segment timer fired and called segmentAbort.abort(), the
running `for await (const event of parseSse(upstream))` stayed
blocked on a `reader.read()` that never resolved. The while loop
never exited, segment_end was never written, controller.close() was
never called, and Netlify eventually pulled the rug somewhere
between ~49 s and ~60 s in.

Fix: parseSse now takes an optional AbortSignal and, when it fires,
calls `reader.cancel()` directly. The WHATWG ReadableStream spec
guarantees that any pending read() resolves with `{done: true}`
after cancel, which lets the for-await exit promptly and the body
loop re-evaluate `while (!done && !segmenting)` → break →
writeJson(segment_end) → controller.close(). Confirmed in the curl
trace: heartbeats now stop at the budget mark and segment_end fires
before Netlify's cap.

Co-Authored-By: alex <alex@semipublic.co>

netlify/edge-functions/agent-proxy.ts

@@ -179,13 +179,30 @@ async function anthropicFetch(
 
 // Parse the SSE stream into an async iterable of decoded events.
 // Each SSE message is `event: <type>\ndata: <json>\n\n`.
+//
+// IMPORTANT: per the Fetch spec, an `AbortSignal` passed to `fetch()`
+// only aborts the request/header phase — once `res.body` is returned,
+// aborting that signal does NOT close the body stream and any
+// in-flight `reader.read()` will hang forever. So we attach our own
+// listener and `reader.cancel()` directly when the caller's signal
+// aborts; the WHATWG spec guarantees pending reads resolve with
+// `{done: true}` after cancel.
 async function* parseSse(
   stream: ReadableStream<Uint8Array>,
+  signal?: AbortSignal,
 ): AsyncGenerator<AgentEvent> {
   const reader = stream.getReader()
   const decoder = new TextDecoder('utf-8')
   let buffer = ''
 
+  const onAbort = () => {
+    reader.cancel().catch(() => {})
+  }
+  if (signal) {
+    if (signal.aborted) onAbort()
+    else signal.addEventListener('abort', onAbort, { once: true })
+  }
+
   try {
     while (true) {
       const { value, done } = await reader.read()
@@ -217,6 +234,9 @@ async function* parseSse(
       }
     }
   } finally {
+    if (signal) {
+      signal.removeEventListener('abort', onAbort)
+    }
     try {
       reader.releaseLock()
     } catch {
@@ -678,7 +698,10 @@ Please follow your instructions to produce the funding outlook brief.`
           // Tail the live stream until upstream EOFs (drop), the
           // segment timer fires, or a terminal event arrives.
           try {
-            for await (const event of parseSse(upstream)) {
+            for await (const event of parseSse(
+              upstream,
+              segmentAbort.signal,
+            )) {
               if (segmenting) break
               if (event.id) {
                 if (!seenEventIds.has(event.id)) {