# 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>
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