fix: unwrap ShellErrorBridge in ByteStream::into_bytes (#16161)

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- closes #16159 

# Description
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The setup in #16159 produced a `ByteStream` that held the `ShellError`
inside an `std::io::Error`. For exactly this setup is the
`ShellErrorBridge` designed, while figuring out what happened I found
out that inside `BytesStream::into_bytes` a sneaky `ShellErrorBridge`
was hiding:
<img width="1034" height="580" alt="image"
src="https://github.com/user-attachments/assets/a0d16ca6-1a60-4c3c-a4cb-5e4b0695f20c"
/>

To fix this, I try to unwrap the bridge and if that is possible, return
the original `ShellError`. This is done at multiple occasions inside
`byte_stream.rs` already.

With this fix, we get the original error that looks like this:
<img width="1439" height="733" alt="image"
src="https://github.com/user-attachments/assets/73f4dee7-f986-4f68-9c2c-0140a6e9e2b2"
/>


# User-Facing Changes
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None, just a better error.

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To have less situations with other I/O errors, I opened #16160 to find
out faster what other error was passed around.

crates/nu-protocol/src/pipeline/byte_stream.rs

@@ -598,7 +598,12 @@ impl ByteStream {
         match self.stream {
             ByteStreamSource::Read(mut read) => {
                 let mut buf = Vec::new();
-                read.read_to_end(&mut buf).map_err(&from_io_error)?;
+                read.read_to_end(&mut buf).map_err(|err| {
+                    match ShellErrorBridge::try_from(err) {
+                        Ok(ShellErrorBridge(err)) => err,
+                        Err(err) => ShellError::Io(from_io_error(err)),
+                    }
+                })?;
                 Ok(buf)
             }
             ByteStreamSource::File(mut file) => {