test(frontend): replace JSON structuredClone polyfill with faithful one

[cristim]

Problem

TEST-07 (docs/reviews/codebase-review-2026-06-10.md): the Jest setup polyfilled structuredClone for jsdom with JSON.parse(JSON.stringify(val)), which drops undefined-valued properties, stringifies Dates, degrades Map/Set to plain objects, and throws on cycles. Tests guarding the deep-copy-state invariant therefore passed against JSON semantics, not browser semantics, so clone tests could stay green while runtime types drifted in the browser.

Fix

• frontend/src/__tests__/setup.ts: keep the existing native-first guard (typeof globalThis.structuredClone === 'undefined'), but when polyfilling, delegate to @ungap/structured-clone, a faithful implementation of the HTML structured clone algorithm. It runs inside the jsdom sandbox, so cloned objects keep sandbox-realm prototypes and instanceof assertions keep working (a Node-realm clone, e.g. a worker_threads MessageChannel round-trip, deserializes with Node-realm prototypes and breaks instanceof Date/Map). Unsupported transfer lists now throw instead of being silently ignored.
• frontend/src/__tests__/ungap-structured-clone.d.ts: typed module declaration (the package ships no types).
• frontend/src/__tests__/utils.test.ts: new deepClone (TEST-07) regression suite asserting real semantics: undefined preservation, Date instance fidelity, Map/Set fidelity, cyclic references, throw on functions. Also removes the stale comment claiming undefined preservation could not be asserted under jsdom.
• frontend/package.json / package-lock.json: add @ungap/structured-clone as a devDependency (it was already in the tree as an eslint transitive dep).

Test evidence

• Pre-fix (JSON polyfill stashed back in): all 5 new regression tests fail (Tests: 5 failed, 86 skipped, 91 total).
• Post-fix full suite: Test Suites: 77 passed, 77 total; Tests: 1 skipped, 2558 passed, 2559 total (the skip is pre-existing).
• npm run typecheck clean; npm run build succeeds.

Closes #1160

Summary by CodeRabbit

• Bug Fixes

  • Improved cloning behavior in the test environment so it matches modern browser semantics more closely, including support for dates, maps, sets, cyclic data, and undefined values.
  • Added safer handling when cloning is used with unsupported transfer options.

• Tests

  • Expanded unit coverage for deep cloning edge cases to prevent regressions.

[cristim]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: f7d424ee-4d1a-405f-8a09-4454123687a6

📥 Commits

Reviewing files that changed from the base of the PR and between 60526ce and df9bd84.

⛔ Files ignored due to path filters (1)

• frontend/package-lock.json is excluded by !**/package-lock.json

📒 Files selected for processing (4)

• frontend/package.json
• frontend/src/__tests__/setup.ts
• frontend/src/__tests__/ungap-structured-clone.d.ts
• frontend/src/__tests__/utils.test.ts

---

📝 Walkthrough

Walkthrough

This change replaces the JSON round-trip polyfill for globalThis.structuredClone in the Jest test setup with the @ungap/structured-clone library, adds corresponding TypeScript type declarations and a devDependency, and adds regression tests validating real structured-clone semantics for deepClone.

Changes

structuredClone polyfill replacement

Layer / File(s)	Summary
Dependency and type declarations frontend/package.json, frontend/src/__tests__/ungap-structured-clone.d.ts	Adds @ungap/structured-clone as a devDependency and declares StructuredCloneOptions and the structuredClonePolyfill default export type.
structuredClone fallback wiring frontend/src/__tests__/setup.ts	Imports structuredClonePolyfill and replaces the JSON-based globalThis.structuredClone fallback with a wrapper delegating to the polyfill, throwing when transfer is requested.
Regression tests for real clone semantics frontend/src/__tests__/utils.test.ts	Updates comments and adds a TEST-07 suite verifying undefined preservation, Date, Map/Set, cyclic references, and function-clone throw behavior.

Estimated code review effort: 2 (Simple) | ~10 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Test as deepClone test
  participant Global as globalThis.structuredClone
  participant Polyfill as structuredClonePolyfill

  Test->>Global: structuredClone(value, options)
  Global->>Global: check options.transfer
  alt transfer provided
    Global-->>Test: throw Error
  else no transfer
    Global->>Polyfill: structuredClonePolyfill(value)
    Polyfill-->>Global: cloned value
    Global-->>Test: cloned value
  end

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	Issue #1160 is addressed, but the AWS IAM console deep link from #39 is not present in the reviewable changes.	Add the AWS IAM console link in the account modal and lock its href, target, and rel attributes with tests.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: replacing the JSON structuredClone polyfill with a faithful implementation.
Out of Scope Changes check	✅ Passed	The changes stay focused on the structuredClone polyfill, its typing, and regression tests, with no clear unrelated additions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/test-07-fix

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[cristim]

Rebased onto origin/main (31 commits ahead).

Conflicts resolved in frontend/package.json and frontend/package-lock.json:

• package.json: main had upgraded @typescript-eslint/eslint-plugin and @typescript-eslint/parser to ^8.0.0 (via the ESLint config PR). Resolution keeps main's ^8.0.0 constraint and adds this PR's @ungap/structured-clone: ^1.3.1.
• package-lock.json: accepted main's lockfile and regenerated via npm install --prefer-offline to incorporate the new dep cleanly.

Gates passed:

• npm run typecheck clean
• npm test: Test Suites: 77 passed, 77 total; Tests: 1 skipped, 2558 passed, 2559 total (all 5 new deepClone regression tests pass)

New head: df9bd84