feat: add subscription.resume endpoint

[sandervanhooft]

Summary

• Adds POST /v1/subscriptions/{id}/resume to reverse a pending cancellation while the subscription is still on its grace period.
• Exposes it as $vatly->subscriptions->resume($id) and $subscription->resume().
• Updates the cancel description in openapi.yaml to point at the new endpoint (was: "Canceled subscriptions cannot be reactivated through the API").

Files

• src/API/Endpoints/SubscriptionEndpoint.php — resume() POSTs to /subscriptions/{id}/resume. Idempotency-Key is auto-attached by VatlyApiClient::performHttpCall() like every other POST.
• src/API/Resources/Subscription.php — $subscription->resume() shortcut on the resource.
• openapi.yaml — new resumeSubscription operation; cancel notes updated.
• docs/Subscriptions.md — new "Resume a subscription" section.
• tests/Endpoints/SubscriptionEndpointTest.php — happy-path + idempotency-key tests.

Test plan

• vendor/bin/phpunit — 96/96 pass (2 new)
• vendor/bin/phpstan analyse — clean
• Manual: verify the actual Vatly API server accepts POST /v1/subscriptions/{id}/resume and returns the resumed subscription

Follow-ups

• vatly-fluent-php PR (depends on this being published): https://github.com/vatly/vatly-fluent-php/pull/new/feat/subscription-resume
• vatly-laravel PR (depends on fluent being published): https://github.com/vatly/vatly-laravel/pull/new/feat/subscription-resume

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