fix: consolidate custom network interface when nic type is preconfigured

[adi-IL]

Fixes #500

What was wrong
When TestVM.UseGVNIC() was called prior to TestVM.AddCustomNetwork(), UseGVNIC() initialized NetworkInterfaces with a single entry containing NicType: "GVNIC" and an empty Network field. Subsequent calls to AddCustomNetworkWithStackType() appended a second network interface rather than configuring the primary NIC. This caused test VMs to spin up with two network interfaces (where NIC 0 defaulted to the default VPC network and NIC 1 had the custom network), leading to failures in environments without a default VPC.

What changed

• Updated AddCustomNetworkWithStackType in fixtures.go for both instance and instancebeta to inspect existing network interfaces. If a single preconfigured NIC without an assigned network exists (such as from a prior UseGVNIC() call), it updates the primary interface with the custom network and preserves the preconfigured NicType.
• Added unit test TestAddCustomNetworkGVNICConsolidation in fixtures_test.go covering UseGVNIC() followed by AddCustomNetwork(), AddCustomNetwork() followed by UseGVNIC(), and beta instance consolidation.

Verification

• go test -v -run TestAddCustomNetworkGVNICConsolidation . -> PASS
• go test . ./cleanerupper/... ./utils/... -> all unit tests PASS
• gofmt -d fixtures.go fixtures_test.go -> clean formatting

[google-oss-prow]

Hi @adi-IL. Thanks for your PR.

I'm waiting for a GoogleCloudPlatform member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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