feat: add x-response-size header to all HTTP responses

[krabat-l]

Summary

• Add a new x-response-size HTTP response header that reports the response body size (in bytes) on every RPC response
• Implemented as a Tower Layer/Service middleware (ResponseSizeLayer / ResponseSizeService), following the same pattern as the existing x-execution-time-ns
  timing header
• Uses http_body::Body::size_hint().exact() to read body size without consuming it

Changes

• bin/debug-trace-server/src/response_size.rs (new): ResponseSizeLayer + ResponseSizeService + ResponseSizeFuture with pin_project_lite
• bin/debug-trace-server/src/main.rs: Chain ResponseSizeLayer as the outermost HTTP middleware wrapping TimingHeaderLayer
• Cargo.toml / bin/debug-trace-server/Cargo.toml: Add http-body dep; bytes + http-body-util as dev-deps

Test plan

• test_response_size_header_set — "hello world" body → header value 11
• test_response_size_empty_body — empty body → header value 0
• test_response_size_large_body — 1 MB body → header value 1000000
• test_format_u64_zero / test_format_u64_max — formatting edge cases