[Bug]: pubsubio imports google.golang.org/genproto/googleapis/pubsub/v1, which was removed from current genproto releasess

### What happened?

Affects: Apache Beam Go SDK v2.75.0 (also confirmed present on master as of 2026-09-04, via code search). Go 1.26 toolchain, but this is independent of Go version — it's a go build/module-resolution failure caused by upstream package removal.

sdks/go/pkg/beam/io/pubsubio/pubsubio.go imports google.golang.org/genproto/googleapis/pubsub/v1 (aliased pb) solely to use pb.PubsubMessage as a plain proto struct for local (un)marshaling — no gRPC client from that package is used. pubsubio.Read/.Write delegate entirely to Dataflow's native beam:transform:pubsub_read/write:v1 external transform, so this import isn't functionally required for either.

Recent google.golang.org/genproto releases no longer contain the googleapis/pubsub/v1 package at all. Any Go module that (a) imports beam/io/pubsubio and (b) needs — directly or transitively — a genproto version released after that package was dropped, fails to build.

Steps to reproduce:

In a Go module, import github.com/apache/beam/sdks/v2/go/pkg/beam/io/pubsubio.

### Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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