## Summary
Fixes#19111
This PR fixes a bug where `fs.createReadStream().pipe(ServerResponse)`
would fail to transfer data when ServerResponse had no handle
(standalone usage). This affected Vite's static file serving and other
middleware adapters using the connect-to-web pattern.
## Root Cause
The bug was in the `ServerResponse.writableNeedDrain` getter at line
1529 of `_http_server.ts`:
```typescript
return !this.destroyed && !this.finished && (this[kHandle]?.bufferedAmount ?? 1) !== 0;
```
When `ServerResponse` had no handle (which is common in middleware
scenarios), the nullish coalescing operator defaulted `bufferedAmount`
to **1** instead of **0**. This caused `writableNeedDrain` to always
return `true`.
## Impact
When `pipe()` checks `dest.writableNeedDrain === true`, it immediately
pauses the source stream to handle backpressure. With the bug,
standalone ServerResponse instances always appeared to need draining,
causing piped streams to pause and never resume.
## Fix
Changed the default value from `1` to `0`:
```typescript
return !this.destroyed && !this.finished && (this[kHandle]?.bufferedAmount ?? 0) !== 0;
```
## Test Plan
- ✅ Added regression test in `test/regression/issue/19111.test.ts`
- ✅ Verified fix with actual Vite middleware reproduction
- ✅ Confirmed behavior matches Node.js
Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
