test(frontend): add unit tests for storage util

[Ma77Ball]

What changes were proposed in this PR?

• Add frontend/src/app/common/util/storage.spec.ts, a new Vitest spec for storage.ts, which previously had no dedicated unit tests.
• Cover the localStorage helpers (localSetObject/localGetObject/localRemoveObject) and sessionStorage helpers (sessionSetObject/sessionGetObject/sessionRemoveObject) across set, get, and remove round-trips.
• Cover the edge branches: absent key returns null, empty-string and other falsy values, and the invalid-JSON parse path that throws.
• Cover jsonCast for well-formed and malformed input, using mocked storage globals so the tests are deterministic and isolated.

Any related issues, documentation, discussions?

Closes: #6619

How was this PR tested?

• Run: cd frontend && node --max-old-space-size=8192 ./node_modules/nx/dist/bin/nx.js test gui --watch=false --include=src/app/common/util/storage.spec.ts, expect all 12 tests passing.
• Test-only change; no production code is modified.

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude Opus 4.8 in compliance with ASF

[github-actions]

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[aglinxinyuan]

LGTM!

[Copilot]

Pull request overview

This PR adds Vitest unit-test coverage for the frontend browser storage utility helpers in storage.ts, addressing issue #6619 by validating expected behaviors for localStorage/sessionStorage wrappers and jsonCast.

Changes:

• Add a new storage.spec.ts covering localSetObject/localGetObject/localRemoveObject and sessionSetObject/sessionGetObject/sessionRemoveObject.
• Add tests for jsonCast success and invalid-JSON throw behavior.
• Use deterministic mocked localStorage/sessionStorage via vi.stubGlobal to isolate tests from the runtime environment.

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