fix(signals): remove usage of SIGNAL from @angular/core/primitives/signals package

[brandonroberts]

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?

• The SIGNAL symbol is imported from the @angular/core/primitives/signals package. This is a private import from Angular. This causes issues syncing the NgRx Signals library into G3.
• Removes readonly from internal signal as this is checked at the type level

Closes #

What is the new behavior?

The SIGNAL symbol is no longer imported from the @angular/core/primitives/signals package.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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