cli: validate command does not report which specific field failed validation in JSON configs

[Haroldwonder]

Problem

run_validate reports whether a config file is valid or invalid but does not indicate which field failed or why. Users must manually inspect the config to find the problem.

Location

src/bin/anchorkit.rs — run_validate

Fix

Use the validate_config_strict.py logic (or port it to Rust) to provide field-level error messages, e.g. 'configs/testnet-example.json: field rpc_url is missing'.