OLS adapter label() returns None for OLS4 term payloads

Tested with `oaklib==0.7.0rc7` and Python 3.12.

`OlsImplementation.label()` returns `None` for GO terms even though the OLS4 response includes a label in the embedded term payload.

## Reproducer

```python
from oaklib import get_adapter
import importlib.metadata as md

print(md.version("oaklib")) # 0.7.0rc7
adapter = get_adapter("ols:go")

print(adapter.focus_ontology) # go
print(adapter.label("GO:0008150")) # None

iri = adapter.curie_to_uri("GO:0008150")
term = adapter.client.get_term(ontology="go", iri=iri)
print(term["_embedded"]["terms"][0]["label"])
# biological_process
```

## Expected behavior

`adapter.label("GO:0008150")` should return the label from the OLS4 term payload, e.g. `biological_process` for GO:0008150.

## Current downstream workaround

In `linkml-term-validator`, I had to wrap label access so that if `adapter.label(curie)` returns `None`, the code checks for an OLS4 response shaped like:

```python
term["_embedded"]["terms"][0]["label"]
```

It seems better for this compatibility with current OLS4 responses to live inside the OAK OLS adapter.

Start with OlsImplementation.label() and reproduce the GO:0008150 case using the issue's Python snippet. Compare its result with client.get_term() and the OLS4 _embedded terms payload; done means adapter.label("GO:0008150") returns biological_process rather than None.

Written by the indexing model from the issue text.