feat(s3): add concurrent chunk prefetch for large file downloads (#8917)

* feat(s3): add concurrent chunk prefetch for large file downloads

Add a pipe-based prefetch pipeline that overlaps chunk fetching with
response writing during S3 GetObject, SSE downloads, and filer proxy.

While chunk N streams to the HTTP response, fetch goroutines for the
next K chunks establish HTTP connections to volume servers ahead of
time, eliminating the RTT gap between sequential chunk fetches.

Uses io.Pipe for minimal memory overhead (~1MB per download regardless
of chunk size, vs buffering entire chunks). Also increases the
streaming read buffer from 64KB to 256KB to reduce syscall overhead.

Benchmark results (64KB chunks, prefetch=4):
- 0ms latency:  1058 → 2362 MB/s (2.2× faster)
- 5ms latency:  11.0 → 41.7 MB/s (3.8× faster)
- 10ms latency: 5.9  → 23.3 MB/s (4.0× faster)
- 20ms latency: 3.1  → 12.1 MB/s (3.9× faster)

* fix: address review feedback for prefetch pipeline

- Fix data race: use *chunkPipeResult (pointer) on channel to avoid
  copying struct while fetch goroutines write to it. Confirmed clean
  with -race detector.
- Remove concurrent map write: retryWithCacheInvalidation no longer
  updates fileId2Url map. Producer only reads it; consumer never writes.
- Use mem.Allocate/mem.Free for copy buffer to reduce GC pressure.
- Add local cancellable context so consumer errors (client disconnect)
  immediately stop the producer and all in-flight fetch goroutines.

* fix(test): remove dead code and add Range header support in test server

- Remove unused allData variable in makeChunksAndServer
- Add Range header handling to createTestServer for partial chunk
  read coverage (206 Partial Content, 416 Range Not Satisfiable)

* fix: correct retry condition and goroutine leak in prefetch pipeline

- Fix retry condition: use result.fetchErr/result.written instead of
  copied to decide cache-invalidation retry. The old condition wrongly
  triggered retry when the fetch succeeded but the response writer
  failed on the first write (copied==0 despite fetcher having data).
  Now matches the sequential path (stream.go:197) which checks whether
  the fetcher itself wrote zero bytes.

- Fix goroutine leak: when the producer's send to the results channel
  is interrupted by context cancellation, the fetch goroutine was
  already launched but the result was never sent to the channel. The
  drain loop couldn't handle it. Now waits on result.done before
  returning so every fetch goroutine is properly awaited.

weed/util/http/http_global_client_util.go

@@ -372,7 +372,7 @@ func ReadUrlAsStream(ctx context.Context, fileUrl, jwt string, cipherKey []byte,
 	var (
 		m int
 	)
-	buf := mem.Allocate(64 * 1024)
+	buf := mem.Allocate(256 * 1024)
 	defer mem.Free(buf)
 
 	for {