ACS doesn't let me create isolated networks with ranges which are not RFC1918

##### ISSUE TYPE

* Bug Report

##### COMPONENT NAME

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API, UI, logic
~~~

##### CLOUDSTACK VERSION

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4.18
~~~

##### CONFIGURATION

N/A

##### OS / ENVIRONMENT

KVM with Tungsten Fabric

##### SUMMARY

I want to create an isolated network with Public IPs which will be then routed with a Tungsten Logical Router.

##### STEPS TO REPRODUCE

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-> Add Network, Isolated. In Gateway place a non-RFC1918 range.
~~~

##### EXPECTED RESULTS

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For the network to be created with that range
~~~

##### ACTUAL RESULTS

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"Virtual Guest Cidr 33.65.45.0/24 is not RFC 1918 or 6598 compliant"
~~~

Reproduce the Add Network → Isolated flow on CloudStack 4.18 with KVM and Tungsten Fabric, using a non-RFC1918 gateway range. Start by tracing the API, UI, and logic validation that produces the RFC 1918 or 6598 error. Done means an isolated network can be created with the requested public range, with appropriate validation preserved for unsupported ranges.

Written by the indexing model from the issue text.