fix: contain AbortError from aborted response body cancellation

`void resp.body?.cancel()` in streamChatEvents' finally floated a
rejected promise: an aborted fetch errors the body with the abort
reason, and cancel() on an errored stream rejects with it, escaping
as unhandledRejection -> uncaughtException and killing pi on Escape.

Await the cancel inside the existing try/catch so the rejection is
actually swallowed. Adds a regression test that aborts a mocked
in-flight stream and asserts no unhandled rejection is raised.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

src/stream.ts

@@ -359,7 +359,7 @@ async function* streamChatEvents(args: {
       // ignore
     }
     try {
-      void resp.body?.cancel();
+      await resp.body?.cancel();
     } catch {
       // ignore
     }