bug: get_dataset_run / get_dataset_runs / delete_dataset_run are unusable on Langfuse v4

Describe the bug

The three dataset-run read helpers on the Langfuse client still call the pre-v4
dataset-run endpoints:

On a Langfuse v4 deployment these paths are rejected, so all three raise
NotFoundError: 404 with the body

This endpoint is not available on deployments running in Langfuse v4 events_only mode.
Learn more about Langfuse v4 at: https://langfuse.com/docs/v4

The same SDK already ships the v4 read client for exactly this data —
client.api.experiments.list() and client.api.experiments.list_items()
(langfuse/api/experiments/raw_client.py:87 and :253) — but no helper uses it.

run_experiment() makes the gap directly visible: it writes its run through
POST /api/public/dataset-run-items, which is accepted on v4, returns a populated
dataset_run_id, and hands the caller a dataset_run_url. That run cannot then be
fetched back with the SDK's own read method.

Steps to reproduce

Against a v4 deployment (LANGFUSE_HOST pointing at it):

import datetime as dt
import time
import uuid

from langfuse import Langfuse

client = Langfuse()
dataset = f"repro-dataset-run-read-{uuid.uuid4().hex[:6]}"

client.create_dataset(name=dataset)
client.create_dataset_item(dataset_name=dataset, input={"q": "hello"}, id=f"{dataset}-item-1")

result = client.run_experiment(
    name="repro-experiment",
    run_name=f"{dataset}-run",
    data=client.api.dataset_items.list(dataset_name=dataset).data,
    task=lambda *, item, **kwargs: "answer",
)
print("run_experiment -> dataset_run_id:", result.dataset_run_id)

try:
    client.get_dataset_run(dataset_name=dataset, run_name=f"{dataset}-run")
    print("get_dataset_run -> OK")
except Exception as exc:
    print("get_dataset_run ->", type(exc).__name__, "status:", getattr(exc, "status_code", None))
    print("  body:", getattr(exc, "body", None))

# The events pipeline is asynchronous, so poll briefly for the run to become readable.
deadline = time.time() + 30
while True:
    experiments = client.api.experiments.list(
        from_start_time=dt.datetime.now(dt.timezone.utc) - dt.timedelta(hours=1),
        name=f"{dataset}-run",
    )
    if experiments.data or time.time() > deadline:
        break
    time.sleep(2)

print("api.experiments.list ->", len(experiments.data), "run(s)")
for experiment in experiments.data:
    print("  ", experiment.id, experiment.name, "items:", experiment.item_count)

client.flush()

Actual behavior

run_experiment -> dataset_run_id: d591fd225d7edec7
get_dataset_run -> NotFoundError status: 404
  body: {'message': 'This endpoint is not available on deployments running in Langfuse v4 events_only mode. Learn more about Langfuse v4 at: https://langfuse.com/docs/v4'}
api.experiments.list -> 1 run(s)
   d591fd225d7edec7 repro-dataset-run-read-6ca29b-run items: 1

The run exists and is readable — just not through the documented helper. Note that
dataset_run_id returned by run_experiment() and the id returned by
api.experiments.list() are the same value, so the data is not lost, only unreachable
from the helper that the SDK exposes for it.

Expected behavior

Either route these helpers through the v4 read path when the deployment is v4
(api.experiments.list / list_items, mapping by id/name and dataset_id), or
state in their docstrings that they are unavailable on v4 and point at
client.api.experiments. Today the docstrings say nothing about v4, so the only signal
a user gets is a 404.

If the first route is taken, the mapping is not a drop-in replacement. Measured against a
v4 deployment for a run written by run_experiment():

and for DatasetRun: id/name/description/metadata/dataset_id come from
Experiment, but dataset_name has no counterpart (only datasetId is returned, so it
needs a datasets.list() lookup), metadata requires fields=metadata, and
startTime/endTime are clipped to the requested fromStartTime window rather than being
the run's createdAt/updatedAt. from_start_time is required and pagination is
cursor-based, whereas the current helpers are page-based.

Additional context

• handle_fern_exception in these helpers only logs; the raised NotFoundError keeps the
  generated dataclass str() (it starts with headers: {...}), so the useful server
  message is only visible via exc.body.
• This is the SDK-side counterpart of langfuse/langfuse#16835, and the two are
  complementary rather than duplicates. #16835 reports that on a non-events_only
  deployment the legacy GET /api/public/datasets/{name}/runs still returns runs created
  via POST /dataset-run-items, while the Experiments UI and GET /api/public/experiments
  do not show them; the open PR langfuse/langfuse#16859 addresses that side. Here the
  deployment is events_only: the legacy read path is rejected outright (404), while
  GET /api/public/experiments returns the run correctly. #16859 touches only
  web/, worker/ and packages/ — no langfuse/ files.
• The rejection itself is intentional on the server side
  (langfuse/langfuse#15313 "reject dataset-run-item surface in events_only mode",
  #15526 "do not read dataset-runs in events_only mode"), which is why this looks like
  an SDK-alignment issue rather than a server regression.