Handle empty upgrade metadata in GetLightdInfo

[Zk-nd3r]

Summary

This replaces the GetLightdInfo empty-upgrades panic path with a failed_precondition service error.

Changes

• add a small helper for selecting the latest validator network upgrade
• use it from both fetch and state backends
• add a regression test for empty upgrade metadata

Context

This is the first single-concern issue/PR pair from the Zaino findings list. Kept narrow on purpose so it can fit your normal review and test policy before I split out the rest.

Testing

• cargo +stable fmt --all --check
• git diff --check

Could not run the targeted Rust test on this Windows host because the local GNU toolchain is missing dlltool.exe; build stops in dependency compile before reaching zaino-state.

Closes #1070

[idky137]

utACK (Thanks for finding and fixing this, and helping to make Zaino stronger!)