add firestore-backed application datastore integration test

[ayushsarode]

What this PR does:

Adds integration test coverage for the Firestore-backed application datastore.

The tests cover core application datastore behavior against Firestore, including creating, retrieving, listing, updating, and deleting application records where applicable. This follows the existing datastore integration test pattern used in the repository.

Why we need it:

Application data is a core part of PipeCD’s datastore layer, and Firestore-specific behavior can differ from unit-test assumptions around document structure, queries, updates, and serialization.

Adding integration coverage helps catch regressions earlier and gives maintainers more confidence that the Firestore implementation stays consistent with the expected datastore contract.

Which issue(s) this PR fixes:

Fixes #7076

Does this PR introduce a user-facing change?:

No.

• How are users affected by this change:
  Users are not directly affected. This is a test-only change that improves confidence in Firestore datastore behavior.

• Is this breaking change:
  No.

• How to migrate (if breaking change):
  Not applicable.

[github-actions]

👋 Hi @ayushsarode, welcome to PipeCD and thanks for opening your first pull request!

We’re really happy to have you here

Before your PR gets merged, please check a few important things below.

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Helpful resources

• Contributing guide: https://github.com/pipe-cd/pipecd/blob/master/CONTRIBUTING.md
• Development setup: https://github.com/pipe-cd/pipecd/blob/master/CONTRIBUTING.md#development

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DCO Sign-off

All commits must include a Signed-off-by line to comply with the Developer Certificate of Origin (DCO).

In case you forget to sign-off your commit(s), follow these steps:

For the last commit:

git commit --amend --signoff
git push --force-with-lease

For multiple commits:

git rebase --signoff origin/master
git push --force-with-lease

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Run checks locally

Before pushing updates, please run:

make check

This runs the same checks as CI and helps catch issues early.

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💬 Need help?

If anything is unclear, feel free to ask in this PR or join us on the CNCF Slack in the #pipecd channel.
You can get your Slack invite from: https://communityinviter.com/apps/cloud-native/cncf

Thanks for contributing to PipeCD! ❤️

[ayushsarode]

@rahulshendre please lmk if this integration test PR adds value