[release-1.15] fix(ipam): preserve IP pool allocation state

[zhangzujian]

Summary

• Backport the IPPool allocation-state fixes and package-level regression tests from fix(ipam): preserve IP pool allocation state #7107.
• Preserve named pool ownership across subnet reconciliation and return removed pool ranges to the default pool.
• Adapt the regression tests to the legacy GetRandomAddress API used by this maintenance branch.
• Exclude the master-only e2e changes intentionally because this branch does not contain test/e2e/kube-ovn/ipam/ipam.go.

Test Plan

• go test -p=1 -count=50 ./pkg/ipam
• go vet ./pkg/ipam
• golangci-lint run --concurrency=2 ./pkg/ipam
• git diff --check origin/release-1.15...HEAD