encoding/json/v2: report EOF for top-level values in UnmarshalDecode

The fully streaming UnmarshalJSONFrom method and UnmarshalFromFunc
introduce an edge case where they can encounter EOF in the stream,
where it should be reported upstream as EOF rather than
ErrUnexpectedEOF or be wrapped within a SemanticError.

This is not possible with other unmarshal methods since the
"json" package would read the appropriate JSON value
before calling the custom method or function.

To avoid custom unmarshal methods from encountering EOF,
check whether the stream is already at EOF for top-level values
before calling the custom method.

Also, when wrapping EOF within a SemanticError, convert it
to ErrUnexpectedEOF to better indicate that this is unexpected.

Fixes #75802

Change-Id: I001396734b7e95b5337f77b71326284974ee730a
Reviewed-on: https://go-review.googlesource.com/c/go/+/710877
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

src/encoding/json/v2/arshal_funcs.go

@@ -9,6 +9,7 @@ package json
 import (
 	"errors"
 	"fmt"
+	"io"
 	"reflect"
 	"sync"
 
@@ -306,6 +307,9 @@ func UnmarshalFromFunc[T any](fn func(*jsontext.Decoder, T) error) *Unmarshalers
 		fnc: func(dec *jsontext.Decoder, va addressableValue, uo *jsonopts.Struct) error {
 			xd := export.Decoder(dec)
 			prevDepth, prevLength := xd.Tokens.DepthLength()
+			if prevDepth == 1 && xd.AtEOF() {
+				return io.EOF // check EOF early to avoid fn reporting an EOF
+			}
 			xd.Flags.Set(jsonflags.WithinArshalCall | 1)
 			v, _ := reflect.TypeAssert[T](va.castTo(t))
 			err := fn(dec, v)