chore: sync with OpenAPI — self-service link operation renames

[sandervanhooft]

Summary

Pulls endpoint paths, method names, and HTTP verbs back in line with the latest OpenAPI spec synced into vatly-docs on 2026-05-21.

This effectively reverses the renames from #14: vatlify's canonical operationIds and URL shapes are the ones the OpenAPI documents, and they remained the spec's source of truth.

Method / URL / verb changes

Before	After
`OrderEndpoint::createInvoiceUpdateLink`	`OrderEndpoint::requestAddressUpdateLink`
`Order::createInvoiceUpdateLink`	`Order::requestAddressUpdateLink`
`POST /orders/{id}/invoice-update-link`	`POST /orders/{id}/request-address-update-link`
`SubscriptionEndpoint::createBillingUpdateLink`	`SubscriptionEndpoint::updateBilling`
`Subscription::createBillingUpdateLink`	`Subscription::updateBilling`
`POST /subscriptions/{id}/billing-update-link`	`PATCH /subscriptions/{id}/update-billing`

Both operations still return a `{href, type}` hosted-link payload — return type stays `Vatly\API\Types\Link`.

Schema cleanup

• `OrderLine::$links` removed. The OpenAPI's `OrderLine` schema has no `links` property — line items aren't top-level addressable resources and don't carry HATEOAS links.
• Unused `OrderLineLinks` class deleted.

Tests

Endpoint tests updated to assert the new URLs and (for `updateBilling`) the PATCH verb. All 71 tests pass.

Breaking changes (downstream coordination required)

This is a breaking-change release. Coordinated companion PRs will update the downstream packages:

• `vatly/vatly-fluent-php`: rename `CreateSubscriptionBillingUpdateLink` action → `UpdateSubscriptionBilling`, plus `SubscriptionHandle` / `Billable` / `BillableFactory` references
• `vatly/vatly-laravel`: only docs + Subscription model legacy method, no API surface

Test plan

• `composer test` → 71/71 pass
• CI green across PHP 8.0-8.4 matrix

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