fix: cap resumable chunk size to request body limit

[rabble]

Summary

• cap advertised resumable chunk size to the configured max request body size before returning /upload/init
• accept RESUMABLE_MAX_REQUEST_BODY_SIZE with UPLOAD_ROUTE_MAX_BODY_SIZE as a compatibility alias and document the contract guard
• add regression coverage for the advertised chunk-size cap and for max-sized resumable chunk uploads

Test Plan

• cargo fmt --manifest-path /Users/rabble/code/divine/divine-upload-server/.worktrees/resumable-contract-alignment/Cargo.toml --check
• cargo test --manifest-path /Users/rabble/code/divine/divine-upload-server/.worktrees/resumable-contract-alignment/Cargo.toml
• python3 -m unittest discover -s /Users/rabble/code/divine/divine-upload-server/.worktrees/resumable-contract-alignment/tests -p 'test_*.py'

Notes

• Pairs with the divine-iac-coreconfig ingress body-limit change already pushed to origin/feat/divine-upload-server-gke.