Move pull.yml to linux_job_v3

45 call sites, all mechanical: the v2 -> v3 rename, EC2 runner labels swapped
for their OSDC equivalents, and the ECR image resolved through
_docker-image.yml's outputs.

test_arm_backend.sh also picks up the pytest-xdist worker cap, since
--numprocesses=auto sized itself from the node's cores and OOMKilled
test-arm-backend-no-driver on an 8 vCPU pod.

Authored with Claude Code.

Author: huydhn