[Asset Inventory][AWS] ELBv2: resolve IPAddresses from DNSName when API returns none

[kubasobon]

Summary

Stacked on #8229 (forward-port of #7277). Review #8229 first; this PR's diff shows only the ELBv2 DNS fallback on top of it.

Problem

entity.Details.IPAddresses is always empty for real-world ALBs and NLBs. The AWS API field AvailabilityZones[].LoadBalancerAddresses[].IpAddress is only populated for NLBs with explicitly assigned Elastic IPs — across 200 sampled internet-facing NLBs it was empty on all of them. The classic ELB (v1) fetcher was given a DNS-resolution fallback in #7277; ELBv2 was not.

Fix

Mirror the ELBv1 pattern in the elb_v2 package:

• elb_v2.go: add unexported hostResolver interface (LookupHost) + resolver field on Provider; set net.DefaultResolver in NewElbV2Provider.
• load_balancer_v2.go: add dnsResolvedIPs []string field; GetIPAddresses() returns it as a fallback when the API slice is empty; add NewElasticLoadBalancerInfo constructor for testability.
• provider_v2.go: after listeners are fetched, if GetIPAddresses() is still empty, resolve DNSName via the injected resolver, sort the result, soft-fail on error (a DNS outage must not fail the fetch cycle).
• provider_v2_test.go: add per-case resolver column with four cases: DNS fallback used + sorted, resolver error soft-fail, API IPs win (resolver not called), and the existing error path.
• fetcher_elb_test.go: switch ELBv2 case to NewElasticLoadBalancerInfo with pre-resolved IPs; assert IPAddresses in expected Details.

Caveat

DNS-resolved IPs are point-in-time. ALB/NLB addresses rotate and vary by resolver vantage point. This matches what the classic ELB fetcher already does and what InfoSec's legacy scripts do — parity, not a new compromise.

Related

• Closes https://github.com/elastic/security-team/issues/18294
• Stacked on: [Asset Inventory][AWS] Forward-port #7277 — EC2/ELB/RDS fields for InfoSec #8229

Checklist

• Tests added (DNS fallback, soft-fail, API-wins paths)
• Mocks updated (mockHostResolver added to elb_v2_mock.go)