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.

[renejeglinsky]

Would that be enough to merge this PR?

Suggested change

	::: warning Deep updates not covered
	::: warning Deep updates not covered in Node.js

[mariayord]

@renejeglinsky It looks like Node.js also supports the same use case as the Java runtime, so we can provide common documentation. Could you please check if the changed docu is ok ?