[ING-239] 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#5582). 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 in the SDK so consumers can set it on create/update and read it from responses:

• Adds grants_target_top_up: Optional[bool] to both RecurringTransactionRule (input) and RecurringTransactionRuleResponse.
• Adds create and update coverage (customer-scoped and top-level) asserting the field is forwarded in the request body and parsed from the response.

No version bump: released separately, per repo convention.

How to try locally

python3 -m pytest tests/test_wallet_client.py tests/test_customer_wallet_client.py