fix: validate DNN static IP before subscriber writes

[carlhus]

fix(webconsole): validate DNN static IP before subscriber writes (#720)

• This PR fixes issue #720 in WebConsole subscriber management.
  It prevents invalid Static IPv4 addresses from being saved when creating or updating subscribers.

• Root cause:

  • Static IP validation was only performed through the optional /verify-staticip request.
  • Subscriber Create and Update APIs did not enforce the same validation before writing subscription data, allowing addresses outside the configured DNN Static IP Pool to be stored.

• Changes:

  • Reuse Static IPv4 validation across Verify and Subscriber Create/Update APIs.
  • Aggregate and cache matching UPF pools; skip pool lookup when no Static IP is configured.
  • Return 400 for invalid addresses and 500 for SMF or pool configuration errors.
  • Pass the current SUPI to avoid self-collision during verification.
  • Extend api_verify_test.go to cover pool aggregation and address validation.

• This is reported in GitHub Issue #720.

[d11nn]

Hi @carlhus,
Please validate static IPs in PatchSubscriberByID as well

The new validation is applied to POST and PUT, but PATCH /subscriber/:ueId/:servingPlmnId still writes SessionManagementSubscriptionData without calling validateSubscriberStaticIPsForWrite.

[d11nn]

@Alonza0314 LGTM