`Arrow.Stream` not working with non-seekable I/O e.g. FIFOs and sockets

I found that `Arrow.Stream` does not work with non-seekable I/O, which should be supported for streaming. Here are some MWEs.

### Named pipes

```bash
mkfifo /tmp/arrow_pipe

# Producer
julia -e '
using Arrow
open("/tmp/arrow_pipe", "w") do io
Arrow.write(io, (i = collect(1:10),); file=false) # streaming
end
' &

# Consumer
julia -e '
using Arrow
open("/tmp/arrow_pipe", "r") do io
for batch in Arrow.Stream(io)
println(batch)
end
end
'
# Result: no output, no error

rm /tmp/arrow_pipe
```

### Sockets

```julia
using Arrow, Sockets

server = listen(9999)
@async begin
conn = accept(server)
Arrow.write(conn, (i = collect(1:10),); file=false)
close(conn)
end

sock = connect(9999)

# This block hangs indefinitely. Press Ctrl-C to proceed.
for batch in Arrow.Stream(sock)
println(batch)
end

wait(t)
# ERROR: TaskFailedException
# nested task error: MethodError: no method matching position(::TCPSocket)
```

### Unix domain sockets

```julia
using Arrow, Sockets

server = listen("/tmp/arrow.sock")
@async begin
conn = accept(server)
Arrow.write(conn, (i = collect(1:10),); file=false)
close(conn)
end

sock = connect("/tmp/arrow.sock")

# This block hangs indefinitely. Press Ctrl-C to proceed.
for batch in Arrow.Stream(sock)
println(batch)
end

wait(t)
# ERROR: TaskFailedException
# nested task error: MethodError: no method matching position(::Base.PipeEndpoint)
```

The above demo should be reproducible with Arrow v2.8.0.

## Diagnosis

The first issue is caused `tobytes(io::IOStream)` unconditionally using `Mmap.mmap(io)`. For named pipes, `filesize(io)` returns `0`, so `Mmap.mmap(io)` silently returns an empty `UInt8[]`.

The rest is caused by `Base.write(io, msg, ...)` calling `position(io)` to record block positions for the file format footer, even when writing streaming format (`file=false`).

I will submit a PR soon.

Reproduce the named-pipe and socket MWEs, then trace Arrow.Stream through tobytes(io::IOStream) and Base.write(io, msg, ...) to see how Mmap.mmap and position(io) are used. Done means Arrow.Stream reads batches from non-seekable pipes and sockets without hanging or raising position errors while preserving streaming behavior.

Written by the indexing model from the issue text.