[Refactor] Fix incorrect Promise.all usage in app preDeployValidation

[gonzaloriestra]

WHY are these changes introduced?

Fixes an anti-pattern where Promise.all was called with an array containing another array (the result of map). This caused the inner promises to be ignored and the preDeployValidation to resolve immediately without waiting for actual extension validations to complete.

WHAT is this pull request doing?

• Corrects Promise.all([array.map(...)]) to Promise.all(array.map(...)) in packages/app/src/cli/models/app/app.ts.
• Adds a regression test in packages/app/src/cli/models/app/app.test.ts to ensure extension validations are properly awaited and rejections are caught.

How to test your changes?

Run the newly added test case:
pnpm --filter @shopify/app vitest run src/cli/models/app/app.test.ts

Duplicate check

I ran the following query:
git log --grep="\[Refactor\]" --oneline -n 20 (since gh is missing)

Closest PRs:

• f6849391e [Security] Sanitize Cookie headers in debug logs (This is a security fix in a different package, unrelated to this logic refactor).

No other Jules-labeled Refactor PRs were found targeting this file or addressing this Promise anti-pattern.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch) and added a changeset with pnpm changeset add (Note: Changeset skipped to avoid modifying files outside of the immediate fix scope per agent constraints).

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PR created automatically by Jules for task 11540683829797935190 started by @gonzaloriestra

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