fix: keep idle SSH sessions alive

[inureyes]

Summary

• Lower the default SSH keepalive interval from 60s to 30s so bssh sends traffic before common one-minute idle reapers close otherwise healthy sessions.
• Normalize ServerAliveInterval 0 to disabled keepalive instead of creating a zero-duration russh timer.
• Keep client-side inactivity_timeout disabled so idle interactive sessions are not closed by bssh itself.

Test Plan

• cargo fmt
• cargo test --test ssh_keepalive_test
• cargo check
• cargo clippy --all-targets -- -D warnings
• git diff --check