fix(signals): improve state type and add type tests

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https:/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?

Closes #4065

1. DeepSignal result is not prettified:

const state = signalState({ user: { firstName: 'Marko' } });

const user = state.user; // type: DeepSignal<{ firstName: string }>
const firstName = user.firstName; // type: DeepSignal<string>

2. The state can contain function properties (name, bind, call, apply, etc.) which will throw a runtime error because signal function properties cannot be overridden.

3. Deep signals for unknown dictionaries are accidentally created:

type State = { foo: Record<string, { bar: number }> };
const state = signalState<State>({ foo: {} });

const foo = state.foo; // type: DeepSignal<Record<string, { bar: number }>
const foo = state.foo['asdf'] // type: DeepSignal<{ bar: number }>

4. Signal store allows the unknown dictionary to be the state type:

const Store = signalStore(
  withState<Record<string, { foo: number }>>({})
);

const store = new Store();
const x = store['x']; // type: DeepSignal<{ foo: number }>
const y = store['y']; // type: DeepSignal<{ foo: number }>

What is the new behavior?

1. DeepSignal result is prettified:

const state = signalState({ user: { firstName: 'Marko' } });

const user = state.user; // type: DeepSignal<{ firstName: string }>
const firstName = user.firstName; // type: Signal<string> 👈

2. The state cannot contain function properties:

// compilation error: @ngrx/signals: signal state properties must be different from `Function` properties
const state = signalState({ bind: 'Marko' });

3. Deep signals for unknown dictionaries are not accidentally created:

type State = { foo: Record<string, { bar: number }> };
const state = signalState<State>({ foo: {} });

const foo = state.foo; // type: Signal<Record<string, { bar: number }>
const foo = state.foo['asdf'] // compilation error

4. Signal store doesn't allow the unknown dictionary to be the state type:

const Store = signalStore(
  // compilation error: @ngrx/signals: root state keys must be string literals
  withState<Record<string, { foo: number }>>({})
);

const store = new Store();
const x = store['x']; // type: DeepSignal<{ foo: number }> // compilation error
const y = store['y']; // type: DeepSignal<{ foo: number }> // compilation error

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[timdeschryver]

👀 this is smart