test: add unit tests for app service

[sherifatolanike]

Summary

Adds focused unit test coverage for src/app.service.ts using NestJS testing utilities and mocked dependencies.

Changes

• Added src/app.service.spec.ts
• Set up the service using Test.createTestingModule
• Mocked required dependencies
• Added success-path tests for every public method
• Added failure and edge-case tests
• Covered invalid input and dependency failure scenarios
• Kept the existing service implementation unchanged

Testing

Ran the AppService Jest test suite and verified that all tests pass under the existing Jest configuration.

Acceptance Criteria

• Every public method has test coverage
• Happy-path behavior is asserted
• Error and edge-case behavior is asserted
• Tests pass successfully under the existing Jest configuration

Closes #1259

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