Add Firestore integration tests for application datastore

What would you like to be added:
Add integration test coverage for the Firestore-backed application datastore, under test/integration/datastore/firestore/application_test.go.

The tests should verify the main application datastore operations against Firestore, such as creating an application, retrieving it by ID, listing applications, updating application configuration, and deleting or disabling application records where applicable. The coverage should follow the existing datastore integration test style used for other Firestore stores in the repository.

Why is this needed:
Application data is one of the core pieces of PipeCD’s datastore layer, but without Firestore integration tests it is harder to catch regressions that only appear when the datastore implementation talks to Firestore.

Unit tests can validate small pieces of logic, but they may miss Firestore-specific behavior such as document structure, query filters, ordering, update semantics, missing records, or serialization differences. Adding these tests would give maintainers more confidence that application datastore behavior works correctly in the real Firestore-backed environment and remains consistent as the implementation evolves.

Start with test/integration/datastore/firestore/application_test.go and compare the existing Firestore datastore integration test style in the repository. Cover creating, retrieving by ID, listing, updating configuration, and deleting or disabling application records where applicable. Run the relevant Firestore integration tests and confirm the main application datastore operations pass against a real Firestore-backed environment.