fix(fetch): throw AbortError from streamResponse instead of swallowing

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Description

Problem

When using the OpenAI provider for autocomplete, moving the cursor during active streaming causes "Malformed JSON sent from server" errors (#7282).

Even though the original issue was closed, we still have a strong rationale for this fix: #7282 (comment).

Root cause

In packages/fetch/src/stream.ts, streamResponse catches AbortError and silently returns. This causes streamSse to fall through to its post-loop buffer flush, which tries to JSON.parse a partial JSON line that was mid-stream when the abort happened:

// streamSse – post-loop code (lines 144-149)
if (buffer.length > 0) {
  const { done, data } = parseSseLine(buffer); // ← partial JSON!
  if (!done && data) yield data;
}

Fix

One-line change in streamResponse (line 52): throw the AbortError instead of swallowing it:

 if (e.name.startsWith("AbortError")) {
-    return; // In case of client-side cancellation, just return
+    throw e; // Let callers handle abort – they must not process partial buffers
 }

The exception propagates through streamSse's for await loop, skipping the post-loop buffer flush entirely. The partial JSON is never parsed.

Impact

• streamSse (29 call sites across providers): protected – post-loop buffer flush is skipped via exception.
• streamJSON (1 call site, Cohere.ts:118): safe – has no post-loop buffer flush to begin with.
• 7 direct streamResponse callers: none have post-loop buffer processing.

Note: I know that changing the behavior from early returning to throwing an exception here has a huge blast radius, but I have checked that the AbortError exception will be properly handled in all direct/indirect call sites, so the change should be safe. I have also manually verified aborting all sorts of operations in Visual Studio Code extension to make sure that I'm not introducing any regression.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

No screen recording. This patch fixes a problem and made the annoying "Malformed JSON sent from server" popup going away.

Tests

Added a test to verify that AbortError was propagated and buffered incomplete data won't be parsed.

We have also tested the built VSIX on Visual Studio Code manually to verify the fix.

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All contributors have signed the CLA ✍️ ✅
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I have read the CLA Document and I hereby sign the CLA

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The jetbrains test failure does not seem to be related to our patch. Everything else looks good.