SPL-299716: M1/app framework v1 app validation

[jzywiecki]

Description

This MR adds validation for App resources to ensure:

• the referenced app source exists before reconciliation proceeds
• duplicate app definitions are rejected so each app remains unique

Key Changes

• Added validation to verify that the referenced AppSource exists
• Added validation to ensure an App is unique
• Added tests covering the new validation behavior

Testing and Verification

• Ran tests locally
• Verified the new validation scenarios works

Alignments with immutability:

• Immutability: scope, spec.appID, spec.targetRef are immutable

Uniqueness: only one App may exist per (targetRef, AppID, scope) and App Source reference

Related Issues

• https://splunk.atlassian.net/browse/SPL-299716

PR Checklist

• Code changes adhere to the project's coding standards.
• Relevant unit and integration tests are included.
• Documentation has been updated accordingly.
• All tests pass locally.
• The PR description follows the project's guidelines.