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