fix(kernel): avoid crashing idle reactive agents

[pandego]

Closes #1102

Summary

• exempt idle reactive agents from heartbeat crash handling when they are not actively processing a turn
• restore any incorrectly crashed idle reactive agent back to Running and reset its recovery tracker state
• add focused heartbeat coverage for reactive-vs-periodic idle behavior

Testing

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace