Limit WebSocket and gRPC request bodies

[ryanfowler]

Summary

• Add a bounded request-body materializer and use it for WebSocket initial messages and unary/raw gRPC request bodies.
• Prevent oversized -d @file or stdin inputs from being fully loaded into memory before existing protocol limits are enforced.
• Add regression tests for oversized file inputs on both paths.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features rejects_oversized_file_before_reading
• cargo test --all-features --lib