Fix type inconsistency in DatasetWrapper method signatures

[Copilot]

The subjectsOf and objectsOf methods used a toTerm helper that accepts string | Term, but their predicate and graph parameters were typed as only string, preventing TypeScript callers from passing Term objects.

Changes:

• Widened predicate and graph parameters to string | Term in both methods
• Aligns with existing object and subject parameters which already accept string | Term

// Before: only strings allowed for predicate/graph
protected* subjectsOf<T>(
  termWrapper: TermWrapperConstructor<T>,
  predicate?: string,
  object?: string | Term,
  graph?: string
): Iterable<T>

// After: consistent Term support
protected* subjectsOf<T>(
  termWrapper: TermWrapperConstructor<T>,
  predicate?: string | Term,
  object?: string | Term,
  graph?: string | Term
): Iterable<T>

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[langsamu]

I think that none of these arguments should be a Term. Choosing strings is an intentional design choice. Like many other design choices, it is not yet documented.