fix(signals): remove signalMethod instance watcher on destroy

[rainerhahnekamp]

Each signalMethod instance uses a watcher assigned to it. The watcher is destroyed when the signalMethod itself gets destroyed. However, watchers were removed prematurely due to onDestroy behavior in effect. • onDestroy doesn’t run when an effect is destroyed but after each execution. • This caused watchers to be removed after the first execution unexpectedly.

Problematic Code: watchers.splice(watchers.indexOf(watcher), 1);

The removal mistakenly affected other watchers, keeping instances running, even after the signalMethod was destroyed.

Co-authored-by: @davdev82

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?

effects are not destroyed.

Closes #4644

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[rainerhahnekamp]

For the record. The not-working destroy is now officially a bug: angular/angular#59410