internal/jsonrpc2: test WireError.Is and fix nil target panic (#1079)

# Problem
WireError.Is backs errors.Is checks used across the SDK against
sentinels such as ErrMethodNotFound, ErrRejected, and ErrClientClosing,
but had no direct test. Add a table-driven test for it.

While doing so, errors.Is(err, (*WireError)(nil)) was found to panic on
a nil pointer dereference, because Is read w.Code without a nil check.
Guard against a nil *WireError target so the comparison returns false.

  ```go
  errors.Is(err, (*WireError)(nil)) // panic: nil pointer dereference
 ```

# Approach


Is read w.Code after a successful type assertion without checking
whether w itself was nil. The fix guards against a nil *WireError target
so the comparison returns false instead of panicking.
# Test plan

  - go test ./internal/jsonrpc2/ passes.
- The new typed nil WireError case panics without the one-line fix and
passes with it, confirming the test exercises the guarded path.

Co-authored-by: Yaroslav <yarolegovich@gmail.com>

internal/jsonrpc2/wire.go

@@ -87,7 +87,7 @@ func (err *WireError) Error() string {
 
 func (err *WireError) Is(other error) bool {
 	w, ok := other.(*WireError)
-	if !ok {
+	if !ok || w == nil {
 		return false
 	}
 	return err.Code == w.Code

internal/jsonrpc2/wire_test.go

@@ -7,6 +7,7 @@ package jsonrpc2_test
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"reflect"
 	"testing"
 
@@ -147,6 +148,29 @@ func TestDecodeIDOnlyMessageIsResponse(t *testing.T) {
 	}
 }
 
+// TestWireErrorIs checks the errors.Is behavior of WireError, including that
+// comparing against a typed-nil *WireError does not panic.
+func TestWireErrorIs(t *testing.T) {
+	err := jsonrpc2.NewError(-32600, "invalid request")
+	for _, test := range []struct {
+		name   string
+		target error
+		want   bool
+	}{
+		{name: "same code", target: jsonrpc2.NewError(-32600, "other message"), want: true},
+		{name: "different code", target: jsonrpc2.NewError(-32601, "invalid request"), want: false},
+		{name: "typed nil WireError", target: (*jsonrpc2.WireError)(nil), want: false},
+		{name: "untyped nil", target: nil, want: false},
+		{name: "other error type", target: errors.New("invalid request"), want: false},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			if got := errors.Is(err, test.target); got != test.want {
+				t.Errorf("errors.Is(%v, %v) = %v, want %v", err, test.target, got, test.want)
+			}
+		})
+	}
+}
+
 func checkJSON(t *testing.T, got, want []byte) {
 	// compare the compact form, to allow for formatting differences
 	g := &bytes.Buffer{}