mask is not applied to create_dataset_item or create_score(comment=)

Describe the bug

mask is documented as the client-level redaction hook (MaskFunction: "A function that masks data"), but it is only applied to span-backed payloads. Two client methods send user-supplied data to the API without ever calling it:

For anyone using mask as a privacy/compliance control, this is a silent gap — the hook is configured once on the client, so the natural reading is that it covers what the client sends.

Evidence (v4.14.1)

_mask is referenced only in _client/span.py and _client/span_exporter.py:

$ grep -rn "_mask" langfuse/ --include='*.py' | grep -v "/span.py" | grep -v _process_media
langfuse/_client/client.py:310:    _mask: Optional[MaskFunction] = None
langfuse/_client/client.py:438:        self._mask = self._resources.mask
langfuse/_client/span_exporter.py:106:        self._mask_otel_spans = mask_otel_spans   # the separate OTel hook

create_dataset_item passes its arguments straight through (client.py:3627):

result = self.api.dataset_items.create(
    dataset_name=dataset_name,
    input=input,                 # not masked
    expected_output=expected_output,
    metadata=metadata,
    ...
)

create_score builds the score body and hands it to add_score_task with comment untouched.

For contrast, create_event is not affected — it constructs a LangfuseEvent (span-backed), so it does go through _process_media_and_apply_mask. The boundary is "span-backed vs direct API call", which is an implementation detail rather than something a caller can infer.

Reproduction

from langfuse import Langfuse

def mask(*, data, **kwargs):
    return "[MASKED]"

lf = Langfuse(public_key=..., secret_key=..., mask=mask)

# masked, as expected
with lf.start_as_current_observation(name="s", as_type="span", input={"x": "secret"}):
    pass

# NOT masked — "secret" is sent verbatim
lf.create_dataset_item(dataset_name="d", input={"x": "secret"})
lf.create_score(name="quality", value=1, comment="secret", trace_id=lf.create_trace_id())

Expected behaviour

Either of these resolves it; I have no strong preference:

1. Apply mask to these payloads too, so the hook means what its name implies. The change in create_dataset_item is a couple of lines against the existing _mask attribute.
2. Document the boundary explicitly — on MaskFunction, on the mask constructor arg, and in the masking docs — stating that mask covers span/generation/event payloads only, and that dataset items and score comments are the caller's responsibility.

Happy to open a PR for (1) if you'd take it — say which of the two you'd prefer and I'll follow the repo's conventions.

Why this came up

We register a masking function so nothing unredacted leaves the process, then discovered our own codebase had grown a comment warning callers that "dataset items go straight to the API and are never masked" — a documented workaround for an undocumented boundary. That seemed worth reporting rather than keeping local.

Environment

• langfuse 4.14.1, Python 3.14, Langfuse Cloud (US)