fix(activator): ip_to_index fn honors ip range

[vihu]

Summary

Fix #2657

This PR addresses the bug by basically ensuring that we call contains method in the ip_to_index helper function; this makes certain that the ip is actually within ranges; without this safety check, when deleting the activator would essentially panic and fail to restart.

Without Fix

    activator_interface_delete_test.go:89:
        	Error Trace:	/home/rahul/malbec-labs/doublezero/e2e/activator_interface_delete_test.go:89
        	Error:      	Should be true
        	Test:       	TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/verify_activator_running
        	Messages:   	activator container is not running - it likely crashed
--- FAIL: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP (181.13s)
    --- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/create_loopback_with_out_of_pool_ip (0.53s)
    --- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/wait_for_interface_activation (14.01s)
    --- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/delete_interface_with_out_of_pool_ip (5.53s)
    --- FAIL: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/verify_activator_running (0.00s)
FAIL
exit status 1
FAIL	github.com/malbeclabs/doublezero/e2e	227.649s
make: *** [Makefile:21: test] Error 1

With Fix

2026-01-17 01:37:41 INF ==> Verifying activator container is still running
2026-01-17 01:37:41 INF --> Activator is still running
=== RUN   TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/verify_interface_removed
2026-01-17 01:37:41 INF ==> Verifying interface is removed from chain
2026-01-17 01:37:49 INF --> Interface successfully removed from chain
--- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP (190.70s)
    --- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/create_loopback_with_out_of_pool_ip (0.53s)
    --- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/wait_for_interface_activation (14.01s)
    --- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/delete_interface_with_out_of_pool_ip (6.04s)
    --- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/verify_activator_running (0.00s)
    --- PASS: TestE2E_ActivatorInterfaceDeleteOutOfPoolIP/verify_interface_removed (8.01s)
PASS
ok  	github.com/malbeclabs/doublezero/e2e	240.147s

[ben-malbeclabs]

LGTM