[codex] Modularize large Rust surfaces

[yvette-carlisle]

Summary

• Split large Rust entrypoint files into normal Rust modules with narrower ownership boundaries.
• Move tests, DTOs, route groups, service logic, storage helpers, and benchmark helpers behind module entrypoints instead of keeping them in monolithic files.
• Preserve existing public surfaces through explicit module exports and visibility where needed.

Validation

• cargo make fmt-rust-check
• cargo make check-rust
• cargo make test-rust (nextest run 1cf97654-ee96-41a5-afde-b58de64fafd6: 370 passed, 92 skipped)
• git diff --cached --check