Summary

Make codespaces RPC setup wait for a real gRPC connection instead of treating lazy client construction as success, so port-forward failures from gh cs ssh surface immediately.

Changes

• replace the lazy grpc.NewClient setup in internal/codespaces/rpc/invoker.go with a blocking grpc.DialContext(..., grpc.WithBlock())
• keep waiting on the same setup race, but only report success once the forwarded local port is actually connected
• add a regression test that reproduces the hidden port-forward error case and asserts the error bubbles up from CreateInvoker

Testing

• go test ./internal/codespaces/rpc/...
• go test ./internal/codespaces/...

Fixes #11206