Go: fix RPC handling of left padded operator enums

[greg-at-moderne]

What's changed?

Fix Go RPC logic to properly handle left-padded operator enums.

What's your motivation?

Panics are raised when running Go recipes in Moderne CLI, e.g.

2026/05/27 11:40:25 PANIC in Print: interface conversion: interface {} is tree.LeftPadded[github.com/openrewrite/rewrite/rewrite-go/pkg/tree.J], not tree.LeftPadded[github.com/openrewrite/rewrite/rewrite-go/pkg/tree.BinaryOperator]
goroutine 1 [running]:
main.(*server).safeHandleRequest.func1()
        rewrite-recipe-workspace/cmd/rpc/main.go:404 +0x84
panic({0x103340820?, 0x140004492f0?})
        runtime/panic.go:783 +0x120
github.com/openrewrite/rewrite/rewrite-go/pkg/rpc.(*JavaReceiver).VisitBinary(0x14000350d40, 0x140003fc1e0, {0x103381700?, 0x1400049f980})
        github.com/openrewrite/rewrite/rewrite-go@v0.0.1/pkg/rpc/java_receiver.go:149 +0x2f0
github.com/openrewrite/rewrite/rewrite-go/pkg/visitor.(*GoVisitor).Visit(0x14000350d40, {0x103436940, 0x140004283c0}, {0x103381700, 0x1400049f980})