fix(kb): gate Knowledge Base creation on every remote backend, not just pgvector (#14686)

fix(kb): gate KB creation on every remote backend, not just pgvector

_validate_create_backend only ran a connectivity probe for postgres and
returned early for every other backend, so a Knowledge Base pointed at an
unreachable OpenSearch or a credential-less Chroma Cloud was created and
persisted (HTTP 201), then indistinguishable from a healthy empty KB until
its first ingestion failed. The Memory Base path already probes every remote
backend and 422s on failure; this brings KB creation into line.

Swap the pgvector-only early return for a local-Chroma early return
(is_local_chroma): local Chroma creates its store lazily and has nothing to
probe, so it is skipped, while every remote backend is connectivity-checked.

Update the three create tests that relied on the early return to mock
test_connection, and add a test asserting an unreachable remote backend is
refused with 422 and leaves nothing persisted.