Read single-subscription events modules back into the TOML subscription list

[dpeacock]

WHY are these changes introduced?

Core is migrating events app modules to a one-module-per-subscription structure (dual contract): each module's config carries events.subscription as a single object instead of a list, with the module uid derived from the subscription handle.

transformToEventsConfig assumes subscription is always a list and calls .map on it, so reading remote configuration (app config link, deploy config diffing) throws a TypeError once an app's stored modules use the new shape.

WHAT is this pull request doing?

• Normalizes the remote events.subscription value to a list (object → one-element list) before stripping the server-managed identifier.
• Multiple single-subscription modules then deep-merge back into one TOML [[events.subscription]] array through the existing remoteAppConfigurationExtensionContent union-array merge — the same mechanism webhook_subscription modules use today.
• Legacy aggregate (list-shape) modules are unaffected; the TOML format does not change.

Known follow-up (out of scope): subscription ordering after merging N remote modules follows server module order, which may produce cosmetic diff noise in app config link output. Webhooks solves this with sort-on-parse (mergeAllWebhooks); events can adopt the same if it becomes a problem.

How to test your changes?

• pnpm vitest run packages/app/src/cli/models/extensions/specifications/transform/app_config_events.test.ts (12 tests, includes single-object wrap + N-module merge)
• Against a top-hat app whose events modules are stored single-shape (see Core PR): shopify app config link round-trips the TOML unchanged.

Measuring impact

• n/a — bugfix/forward-compat for internal storage migration

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes