[ING-240] feat(wallets): add grants_target_top_up to recurring transaction rules

[aquinofb]

Context

Wallet recurring transaction rules of type target previously always created a paid top-up; there was no way to grant free credits through a target rule. The API now supports this via a nullable boolean grants_target_top_up on the recurring transaction rule (getlago/lago-api#5755). On a target rule, true makes the periodic gap-fill grant free credits to reach the target balance instead of creating a paid top-up; false, the default for target rules, keeps the current paid behaviour. The field is null for fixed rules.

Description

Exposes grants_target_top_up so consumers can set it on wallet create/update:

• Whitelists grants_target_top_up in the recurring transaction rule params. Responses already surface it (parsed dynamically into OpenStruct), so no response-model change is needed.
• Adds create and update specs asserting the field is forwarded on a target rule.

No version bump: released separately, per repo convention.

How to try locally

bundle exec rspec spec/lago/api/resources/customers/wallets_spec.rb