clarify @mandatory is not validated in deep updates

[mariayord]

Updated documentation regarding @mandatory annotation. There is no validation on @mandatory in deep updates usecase.

Removed section with @FieldControl, as it is mentioned already in Field Control below

Fixes: https://github.tools.sap/cap/cdsnode/issues/2637

[renejeglinsky]

@johannes-vogel @agoerler could you have a quick check here?

[renejeglinsky]

@rjayasinghe Have you reviewed this PR?

[rjayasinghe]

The changes in this PR reflect the Node.js runtime — they do not hold for CAP Java.

The statement "@mandatory validation is not applied to nested data in deep update operations" is accurate for Node.js, but as written it reads as cross-runtime and is misleading for Java. Since these docs cover both runtimes, could we scope it per-runtime?

In CAP Java, @mandatory validation (MandatoryHandler → NotNullValidator) does recurse into nested data on deep updates, following the same cascade rules as persistence — i.e. into compositions by default (and any association carrying @cascade.update/@cascade.all). The only relaxation is partial-update semantics. Concretely, for a deep update:

Nested @mandatory field	CAP Java behavior
omitted from the payload	tolerated (partial-update semantics)
explicit null / "" in a composition	rejected (VALUE_REQUIRED)
explicit null / "" in an association target	tolerated

So for Java it's not "no validation on nested data" — it's "a missing mandatory value is not enforced on update, but an explicitly provided null inside a cascading composition still is."

This is backed by the unit tests in MandatoryHandlerTest (the composition-null case is asserted to fail; I've added coverage for the omitted-field case confirming it's tolerated).

Suggested framing: keep the current sentence under a Node.js tab, and for Java state that mandatory is validated in nested cascading targets, with only a missing value left unenforced (use not null if you also need presence enforced).

[mariayord]

@rjayasinghe As I understand it, Java does not validate @mandatory, except for compositions where the value is explicitly set to null or ''.
@renejeglinsky In that case, I will document this as the Node.js behavior.
@rjayasinghe For Java, it's up to you whether you want to document this partially supported feature.