Search all ranges in a pool for the next external IP

[bnaecker]

• Add a where-clause to ensure we skip IP Pool Ranges which have been exhausted when searching for the next external IP address.
• Add regressions
• Fixes Insufficient external IP address declared prematurely #9342

[david-crespo]

One test failure due to the internal error message changing:

thread 'db::queries::external_ip::tests::test_ephemeral_and_snat_ips_do_not_overlap' panicked at nexus/db-queries/src/db/queries/external_ip.rs:1129:9:
  assertion `left == right` failed
    left:  InsufficientCapacity { message: MessagePair { external_message: "No external IP addresses available", internal_context: "NextExternalIp::new returned NotFound" } }
    right: InsufficientCapacity { message: MessagePair { external_message: "No external IP addresses available", internal_context: "NextExternalIp::new tried to insert NULL ip" } }