Improve error handling: check installApp return value before proceeding to initialization

Context

The installApp method in packages/apps-engine/src/server/AppManager.ts returns a Promise<boolean> to signal success or failure. When an exception occurs during installation, it returns false and sets the app status to ERROR_DISABLED.

However, in the add() function (around line 636), this return value is currently ignored, allowing the app to proceed to initializeApp or runStartUpProcess regardless of whether installation succeeded.

Problem

This behavior is inconsistent with error handling elsewhere in the class and may mask critical installation failures. The return value should be captured and failures should be handled appropriately—either by preventing app initialization/enablement or by explicitly documenting why errors are non-blocking.

Related

• PR: https://github.com/RocketChat/Rocket.Chat/pull/37167
• Discussion: https://github.com/RocketChat/Rocket.Chat/pull/37167#discussion_r2511388279
• Requested by: @d-gubert

This issue was identified during code review but marked as out of scope for the original PR.