Fix gRPC dry-run stdin reuse

[ryanfowler]

Summary

• Materialize dry-run request bodies once for stdin-backed gRPC streams and reuse the result for output.
• Preserve the existing preview path for non-streaming bodies while routing binary decisions through a shared helper.
• Add a regression test covering client-streaming gRPC dry-run with stdin input.

Testing

• cargo test --all-features --test grpc grpc_dry_run_materializes_streaming_stdin_body_once
• cargo test --all-features --test grpc