fix(ti): send service_specification_ref in TI activate command

SRM rejects every Traffic Influence activate command because
network_capability.target.application_reference.service_specification_ref
is required by its ApplicationReferenceTarget model.

build_activate_command now sets service_specification_ref to the appId,
matching the spec ref build_catalog_payload registers on POST /apps.
external_app_id is kept.

The parameters_snapshot read path gets its own target/application_reference
models where service_specification_ref is optional, so snapshots stored
before this change still load on GET.