Defer WebSocket dry-run body materialization

[ryanfowler]

Summary

• Move WebSocket initial-message construction until after the dry-run early return so --dry-run no longer reads stdin or large request bodies.
• Add a regression test that keeps stdin open with -d @- and verifies dry-run exits without blocking.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features --test websocket websocket_dry_run
• cargo test --all-features