fix: preserve DoH TTL presence

[ryanfowler]

Summary

• preserve missing DoH TTLs as unavailable during DNS inspection
• render supplied zero TTLs as 0s and retain existing nonzero formatting
• cover missing, zero, and nonzero TTL output

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins
• full requested integration test suite with two test threads