fix: vehicle-speed-attribute.json (CP stub reply) must carry its own @context

This fixture is served as the mock Context Source's GET/query reply in the
distributed retrieve/query tests (D010, D011) — a forwarded NGSI-LD response,
which a conformant consumer interprets with the context that travels WITH it
(§ 6.3.5), core if none. Without @context the broker correctly expands the CP's
'speed' with core only → the default-context IRI, not the suite's
https://ngsi-ld-test-suite/context#speed → the attribute is dropped on merge.

Add @context (the test-suite compound context) so the mock is a conformant CP.
This fixture is stub-reply-only — request payloads use the separate .jsonld
variant — so embedding @context here is safe. Distop subtree 29→22 fails, the
2 unclear D009 gone. Some D011 query tests still fail on a separate issue
(Load-As-Serialized-Array / projection), tracked next.