[SPARK-58413][SQL][TEST] Rename sequential fetch label to pipelined fetch (1 client) in NettyTransportBenchmark

### What changes were proposed in this pull request?

This PR renames the benchmark case label `sequential fetch` to `pipelined fetch (1 client)` in the `File-Backed Shuffle Block Fetch` suite of `NettyTransportBenchmark`.

### Why are the changes needed?

The label is misleading. The case fires all 100 `fetchChunk` requests at once on a single connection and then waits for all of them to complete via a semaphore (`fetchChunksSync`), so the client never waits for one chunk before requesting the next. This is a pipelined fetch over one connection, not a sequential request-response loop.

The mislabel skews the interpretation of the results. For example:

https://github.com/apache/spark/blob/12785d56624a8b412e78559dd920b816d629704b/core/benchmarks/NettyTransportBenchmark-results.txt#L132-L133

Read as "sequential vs parallel", the 1.8X looks like poor parallelization efficiency. The actual comparison is "pipelined over 1 socket vs pipelined over 4 sockets", where the single pipelined connection already runs at ~4.2 GB/s. The new label makes this clear and contrasts naturally with `parallel fetch (4 clients)`.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Label-only change; manually reviewed.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Fable 5

Closes #57616 from dongjoon-hyun/SPARK-58413.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

core/src/test/scala/org/apache/spark/network/NettyTransportBenchmark.scala

@@ -599,7 +599,7 @@ object NettyTransportBenchmark extends BenchmarkBase {
           output = output)
 
         Seq("NIO", "AUTO").foreach { mode =>
-          benchmark.addTimerCase(s"$mode, sequential fetch", numIters = 3) { timer =>
+          benchmark.addTimerCase(s"$mode, pipelined fetch (1 client)", numIters = 3) { timer =>
             val conf = createConf(mode)
             val streamManager = createFileStreamManager(conf, files)
             val rpcHandler = createStreamRpcHandler(streamManager)