[WIP] chore(backend): Storage-layer refactor & Postgres enablement roadmap

[kaikaila]

Background

Kubeflow Pipelines currently hardcodes MySQL assumptions in its storage layer. To enable PostgreSQL adoption (via pgx driver) and improve long-term maintainability, we need to decouple SQL dialect logic and centralize it into a reusable abstraction.

⸻

Changes

This PR introduces a major refactor of the backend storage layer and related test workflows:
Dialect abstraction
• Introduced DBDialect interface and refactored pipeline/experiment/run/job stores to use it.
• Removed legacy SQLDialect, centralized initialization in InitDBClient.
Storage refactor
• Updated resource_reference, db_status_store, and cache/storage clients to use the new dialect.
• Moved filter logic into dialect package, cleaned up list package.
• Adapted db_fake to support dialect abstraction.
Tests & dependencies
• Updated unit tests across storage, list, and filter packages.
• Updated go.mod dependencies and tidied modules.
• Added PostgreSQL GitHub Actions workflow with kustomize deployment and port-forward.

⸻

CI Status

• In the upstream CI, the failures are infra-related: tests share the same database and pollute each other.
• Running the tests in parallel jobs resolves this issue and results in all tests passing, as shown in this PR's CI results.
• Per maintainer feedback, PR#12251 was closed to make room for the ongoing test overhaul PR#12222.
• This PR includes the fixes from PR#12251, so CI results here are green.

[google-oss-prow]

Hi @kaikaila. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[github-actions]

🚫 This command cannot be processed. Only organization members or owners can use the commands.