Bound dry-run request body previews

[ryanfowler]

Summary

• Added a bounded dry-run preview path that prints at most 1024 bytes and emits a truncation warning when the preview is cut off.
• Removed the old dry-run behavior that could fully materialize stdin, gRPC, file, and multipart bodies just to inspect them.
• Added multipart preview support that reads file parts only as far as needed for the preview cap.
• Added regression coverage for large @file, @-, and multipart file bodies.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1