Fix cloning errors for objects with custom JSON serialization

[Copilot]

TypeScript 4.8+ strict mode throws when ClonerService attempts to clone objects like Dayjs or Moment. These objects have toJSON() methods that return ISO strings, causing JSON.parse(JSON.stringify()) to produce primitives. The subsequent fixTypes() call then attempts property assignment on strings, triggering: Cannot create property '$d' on string.

Changes

Detection logic via isCloneable():

• Objects with toJSON() returning primitives → structural sharing
• Objects with prototype methods beyond constructor → structural sharing
• Plain objects and arrays → deep cloned (unchanged behavior)
• Simple data classes (properties only) → deep cloned (unchanged behavior)

Updated cloning flow:

• deepClone(): checks cloneability before JSON serialization
• fixPropertyValue(): preserves complex object references, guards against primitive assignment

Example:

const state = {
  user: 'John',
  createdAt: dayjs('2020-01-01'),
  settings: { theme: 'dark' }
};

const cloned = cloner.deepClone(state);
// state.settings !== cloned.settings (deep cloned)
// state.createdAt === cloned.createdAt (reference preserved)
// cloned.createdAt.format('YYYY-MM-DD') works correctly

This enables Angular 14→15+ migrations for projects using date libraries with custom serialization.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Deep cloning process not compatible with complex object (such as Dayjs or Moment for example)</issue_title>
<issue_description>While upgrading Angular from 10 to 15 on a project using this lib, I encountered a problem migrating from Angular 14 to Angular 15. Due to TypeScript upgrade, and the fact that TypeScript 4.8+ is more strict than TS 4.7, it caused runtime errors while using the ObservableState on a state containing Dayjs instances.

The problem is that in the "fix types" process, the cloner service fixes some object types that don't support well the use of JSON.parse(JSON.stringify()) to create the initial copy. But complex types such as Dayjs (or any other type which prototype possess a toString method, I guess) are not fixed properly.

This causes runtime errors like this :

TypeError: Cannot create property '$d' on string '2019-12-31T23:00:00.000Z'
    at ClonerService.fixPropertyValue (http://localhost:9876/_karma_webpack_/webpack:/node_modules/@codewithdan/observable-store/dist/utilities/cloner.service.js:48:30)
```</issue_description>

<agent_instructions>Research this issue and identify a solution that's still high performance yet can handle the  more complex object types. Consider any additional edge cases to consider as well. Ensure you spend time exploring and thoroughly understanding the repository code first.  </agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Deep cloning process not compatible with complex object (such as Dayjs or Moment for example) #314

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[DanWahlin]

Superseded by #324, which takes a different approach: instead of preserving references to complex objects (which breaks mutation isolation — the core promise of a state management library), #324 actually clones them using a safe strategy chain (clone() → constructor → Object.create → reference fallback).

See the comparison table in #324 for full details. The key difference: with #315, mutating a Dayjs object from getState() would leak back into the store's internal state. With #324, clones are fully independent.

22 new tests cover Dayjs/Moment-like objects, arrays, mutation isolation, mixed types, and edge cases. All 59 specs pass.