feat(api): add sandbox delivery contracts

[bjarn]

Summary:

• regenerate Rust types from the current Sending and Team specifications
• model nullable single-send message IDs and strict batch message IDs
• add sandbox request, response, recipient, and builder coverage
• use the documented cancel response type

Validation:

• cargo +1.98.0 fmt --all -- --check
• cargo +1.98.0 test --all-targets (21 passed)
• cargo +1.98.0 clippy --all-targets --all-features -- -D warnings