fix: handle direct lease extension results

[ccarvalho-eng]

What changed

• Handle Bedrock Repo.transact/1 returning the callback result directly in LeaseExtender.
• Update LeaseExtender tests so the mock transaction contract matches Bedrock's actual transaction return shape.

Related: bedrock-kv/bedrock#108
Found while validating dark-trench/squidie#392 for dark-trench/squidie#384 with the Squidie Bedrock sample app multi-node workflow QA.

Why

Squidie multi-node Job Queue QA exposed LeaseExtender crashes when Store.extend_lease/4 returned {:ok, %Lease{}} directly from Repo.transact/1; the extender was still matching the older nested {:ok, {:ok, lease}} shape.

Validation

• Job Queue full test suite passed.
• Squidie Bedrock sample QA passed for multi-node workflow delivery against local latest Bedrock and Job Queue checkouts.