netConn and msgReader should return io.EOF identically instead of wrapped

[*bufio.Scanner](https://pkg.go.dev/bufio#Scanner) and other consumers of `io.Reader` interface expect to receive `io.EOF` identically when reading to the end of a file/stream.

From the io package:

> // EOF is the error returned by Read when no more input is available.
> // (Read must return EOF itself, not an error wrapping EOF,
> // because callers will test for EOF using ==.)
> // Functions should return EOF only to signal a graceful end of input.
> // If the EOF occurs unexpectedly in a structured data stream,
> // the appropriate error is either [ErrUnexpectedEOF] or some other error
> // giving more detail.
> var EOF = errors.New("EOF")

We currently wrap `EOF` along with any other error when reading from `netConn` or `msgReader`. Instead, we should return `io.EOF` identically to conform with the `io.Reader` spec.

Locate the netConn and msgReader implementations and inspect how they report errors at end of input. Compare their behavior with the io.Reader requirement that EOF be returned identically, then verify that both readers return io.EOF directly rather than a wrapped error.

Written by the indexing model from the issue text.