Fix webhook handler dispatch with shopify_api v16

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What this PR does

Update generated webhook jobs to implement the shopify_api v16
WebhookHandler dispatch contract:

def self.handle(data:)
  perform_later(
    topic: data.topic,
    shop_domain: data.shop,
    webhook: data.body,
  )
end

Generated jobs previously used the pre-v16
handle(topic:, shop:, body:, webhook_id:, api_version:) signature.
shopify_api v16 dispatches WebhookMetadata through handle(data:), so
registered webhook handlers raised ArgumentError during delivery.

This also conforms the webhook fixtures used across the test suite and adds
regression coverage through the real Registry dispatch path.

Fixes #2045

Reviewer's guide to testing

1. Run bundle exec rake test.
2. The regression test in test/shopify_app/managers/webhooks_manager_test.rb
   registers a job through WebhooksManager.add_registrations, then dispatches a
   real HMAC-signed webhook via ShopifyAPI::Webhooks::Registry.process, asserting
   the job is enqueued with the mapped topic / shop_domain / webhook arguments.
3. Generator tests for add_webhook, add_app_uninstalled_job, and add_privacy_jobs
   now assert that generated jobs implement the v16 handle(data:) contract.

Things to focus on

1. The generated handle(data:) contract and the data.topic / data.shop / data.body mapping.
2. The regression test exercises the real Registry dispatch path rather than the mocked registration.
3. CHANGELOG.md and docs/Upgrading.md examples updated to the v16 contract.

Checklist

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in /docs, if necessary
• For security fixes, the Disclosure Policy must be followed.

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