fix(data): make non-optimistic add command entity partial

[Nosfistis]

Mirror the type definition of EntityCollectionServiceBase.add on EntityServerCommands.add.

Closes EntityServerCommands.add does not allow optional id #3847

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/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?

EntityServerCommands.add() allows only full entities.

Closes #3847

What is the new behavior?

EntityServerCommands.add() allows partial entities if non-optimistic, just like EntityCollectionServiceBase.add().

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[timdeschryver]

Could you also add a test that verifies this functionality please?

[Nosfistis]

Given that this is just a type change, I found it not necessary. I will add a test that would cause Typescript to fail build if it does not work.

[Nosfistis]

Digging a little deeper in the dispatcher I found out that the default behavior is indeed pessimistic. However, the initial PR types have been reverted, and omitting the options argument causes a non-partial entity argument (optimistic-like).

I suggest using

add(
    entity: Partial<T>,
    options?: EntityActionOptions & { isOptimistic?: false }
  ): Observable<T>;
  add(
    entity: T,
    options: EntityActionOptions & { isOptimistic: true }
  ): Observable<T>;
  add(entity: T, options?: EntityActionOptions): Observable<T>;

which seems to cover all cases without fail.

[timdeschryver]

👍 thanks