fix(signals): expose readonly signals in slices

[rainerhahnekamp]

withState, signalState, and withLinkedState now pass a Signal instead of the original WritableSignal to the DeepSignal.

This prevents accidental misuse where consumers (e.g. ngModel or other APIs with incompatible types) could assert WritableSignal and write directly into the state.

Closes #4958

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

const person = signalState({ title: 'Title'});
(person.title as WritableSignal<string>).set('Hallo'); <-- direct access to state

Closes #4958

What is the new behavior?

Will throw an error if set is called because the Signal is readonly.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[markostanimirovic]

Let's merge this in v21 to prevent unintentional breaking changes.