Fix UAF in batch_create_stream_feedback_race unittest

[chenBright]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

StreamingRpcTest.batch_create_stream_feedback_race crashes
with SIGSEGV inside Stream::Consume(ci1 ci2):

#0  0x00007ff22a810461 in brpc::Stream::Consume (meta=0x2757c540, iter=...) at src/brpc/stream.cpp:604
#1  0x00007ff22a69453c in bthread::ExecutionQueueBase::_execute (this=this@entry=0x2757e300, head=<optimized out>, high_priority=<optimized out>, niterated=niterated@entry=0x0) at src/bthread/execution_queue.cpp:308
#2  0x00007ff22a6948e0 in bthread::ExecutionQueueBase::_execute_tasks (arg=<optimized out>) at src/bthread/execution_queue.cpp:169
#3  0x00007ff22a6bf510 in bthread::TaskGroup::task_runner (skip_remained=<optimized out>) at src/bthread/task_group.cpp:388
#4  0x00007ff22a693b21 in bthread_make_fcontext () from ./libbrpc.dbg.so
#5  0x0000000000000000 in ?? ()

Stream close is asynchronous: StreamClose only marks the fake socket
failed; the handler's on_closed is invoked later on the consumer
bthread, when Consume is invoked with iter.is_queue_stopped().

The test allocates BatchStreamClientHandler and
BatchStreamFeedbackRaceState on its stack and passes the handler
pointer to two streams via StreamOptions::handler. It also keeps a
SocketUniquePtr (client_extra_ptr) alive across the whole test body
to enlarge the SetConnected race window. That reference pins the
extra stream's fake socket, so BeforeRecycle ->
execution_queue_stop -> the final Consume(is_queue_stopped) for the
extra stream cannot run until client_extra_ptr itself destructs at
scope exit — which happens after the handler/state stack objects have
already been destroyed. When Consume finally runs,
_options.handler is dangling.

Other tests in this file avoid the issue by setting a flag in
on_closed and waiting on it before returning, but
BatchStreamClientHandler::on_closed was empty.

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
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