fix: use JSON.pretty_generate (pretty_unparse removed in json 3.0)

[aim2bpg]

JSON.pretty_unparse was removed in json 3.0.0.rc1 (ruby/json CHANGES.md). Under json >= 3.0, any unhandled exception reaching LambdaLogger.log_error crashes with NoMethodError instead of reporting the original error.

pretty_generate is the documented replacement, present since json 0.4.2 (2006), so this works under json 2.x and 3.x alike.

Repro:

require 'json'
JSON.pretty_unparse({ a: 1 }) # NoMethodError under json 3.0