Prepare version 3.1.0, add Trust API

[barnett]

Fixes #66

Prepare the PHP SDK for a non-breaking endpoint coverage release.

Summary

• Bump SDK minor version to 3.1.0.
• Add Trust verification gateway helpers and trust alias.
• Add documented endpoint helpers/path construction so SDK surface audit covers all public API operations.
• Add top-level payment lookup support.
• Fix RecipientAccountGateway::update() to return RecipientAccount instead of Recipient.
• Add documented response attributes across batch, payment, recipient, recipient account, offline payment, and invoice payment models.
• Add PHP 8.1 iterator compatibility to ResourceCollection without dropping existing PHP version compatibility.
• Set default configuration environment to production.

Open PR/issue review

• Included: fix issue for update in RecipientAccountGateway.php #66 recipient account update return type.
• Partially included: adding php81 compatibility #65 PHP 8.1 compatibility via non-breaking iterator compatibility; did not raise minimum PHP to 8.1.
• Included: Default $_environment should be ready for production #61 production default environment behavior.

Compatibility notes

• Existing public methods remain available.
• New coverage is additive through helpers, aliases, and model fields.
• Minimum PHP requirement remains >=5.4 to avoid breaking existing users.

---

Note

Medium Risk
Pagination and default production environment change runtime behavior for some callers; changes are mostly additive but affect how collections fetch next pages and which API host is used when env was unset.

Overview
Releases 3.1.0 as an additive API-coverage update: new Trust/verification gateway (verification() and trust()), balance helpers (all, paymentrails, paypal), top-level PaymentGateway::find, and recipient offline payment listing with paging.

Pagination and collections are tightened so foreach over results keeps the right path context (batchId, balance segment, recipient id) and only auto-pages when a pager is configured; PHP 8.1 iterator compatibility is added via #[\ReturnTypeWillChange] without raising the minimum PHP version.

Models gain documented API fields (e.g. batch quoteExpiredAt/tags, expanded Payment withholding/return fields, invoice payment and recipient account fields). RecipientAccountGateway::update now returns a RecipientAccount. Default Configuration environment is production. Unit tests cover pagination and model defaults.

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[heyflowmo]

looks good

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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[heyflowmo]

looks good.