[ING-238] feat(wallets): document grants_target_top_up on 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 (and bypasses the paid top-up minimum) 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

Documents grants_target_top_up in the OpenAPI spec so consumers and generated SDKs reflect it:

• Adds the field to the recurring transaction rule schema (WalletRecurringTransactionRule) and to the inline recurring rule definitions on the wallet create and update inputs.
• Models it as a nullable boolean (type: [boolean, "null"]), matching the other nullable fields on the schema.
• Marks it required on the response schema, consistent with the other always-serialised fields there (e.g. ignore_paid_top_up_limits, started_at), since the API always returns the key.
• Regenerates the bundled openapi.yaml.

No spec version bump: handled separately at release time, per repo convention.

How to try locally

npm run build && npm test

Then confirm the field renders on the wallet recurring transaction rule create / update / response schemas.