feat: Add service mapping and defensive global region check for v2 Auth Eventarc triggers

[shettyvarun268]

This PR introduces underlying CLI service mapping and defensive trigger region validation for the upcoming v2 Auth Eventarc triggers (google.firebase.auth.user.v2.created and google.firebase.auth.user.v2.deleted). This work is designed to operate directly in tandem with the Auth Eventarc integration currently being developed in the firebase-functions SDK.

During the SDK design review, it was noted that Firebase Auth Eventarc events operate globally and strictly require their Eventarc trigger location to be configured as region: "global". While the Node.js SDK populates this by default, this PR adds a defensive validation layer in the CLI to guarantee that region: "global" is consistently enforced during deployment across all current and future SDKs. To achieve this, this PR officially registers AUTH_EVENTS within the v2 events registry, implements a dedicated authEventarcService helper to defensively verify and override the trigger region to "global", and maps the new event types accordingly. This approach mirrors the established architectural patterns used for other global event services in the CLI, such as Firebase Test Lab and Firebase Alerts.

[gemini-code-assist]

Code Review

This pull request adds support for Firebase Auth Eventarc triggers, including the registration of the 'autheventarc' service, mapping of the v2 user created and deleted events, and validation to ensure the trigger region is set to 'global'. The review feedback suggests refactoring the region validation function to be async rather than manually returning Promise.resolve(). Additionally, the unit tests should be updated to deep-copy the mock endpoint's nested eventTrigger object to prevent shared state mutation and potential test flakiness.