aws_instance: fix ipv6_addresses / ipv6_addresses_count refresh

[justinretzolk]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

No changes

Description

Updates aws_instance so that ipv6_addresses are recomputed when ipv6_address_count is updated.

In order to achieve this, I needed to remove ForceNew from ipv6_addresses, but reimplemented the functionality via customdiff.ForceNewIf.

The TestAccEC2Instance_IPv6AddressesExplicit test may be overkill -- since the instance is recreated when ipv6_addresses is explicitly changed, ipv6_address_count should inherently be recomputed since it's a new instance. I introduced it anyways, to make sure that the customdiff.ForceNewIf did what I was expecting.

I also updated the name of the TestAccEC2Instance_IPv6AddressCountAndSingleAddressCausesError test function in order to make it more convenient to run with TESTS=TestAccEC2Instance_IPv6.

Relations

Closes #43009

Output from Acceptance Testing

$ make testacc TESTS=TestAccEC2Instance_IPv6 PKG=ec2
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.24.4 test ./internal/service/ec2/... -v -count 1 -parallel 20 -run='TestAccEC2Instance_IPv6'  -timeout 360m -vet=off
2025/06/24 10:04:41 Creating Terraform AWS Provider (SDKv2-style)...
2025/06/24 10:04:42 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccEC2Instance_IPv6_supportAddressCount
=== PAUSE TestAccEC2Instance_IPv6_supportAddressCount
=== RUN   TestAccEC2Instance_IPv6AddressCountAndSingleAddressCausesError
=== PAUSE TestAccEC2Instance_IPv6AddressCountAndSingleAddressCausesError
=== RUN   TestAccEC2Instance_IPv6_primaryEnable
=== PAUSE TestAccEC2Instance_IPv6_primaryEnable
=== RUN   TestAccEC2Instance_IPv6_primaryDisable
=== PAUSE TestAccEC2Instance_IPv6_primaryDisable
=== RUN   TestAccEC2Instance_IPv6_supportAddressCountWithIPv4
=== PAUSE TestAccEC2Instance_IPv6_supportAddressCountWithIPv4
=== RUN   TestAccEC2Instance_IPv6AddressCount
=== PAUSE TestAccEC2Instance_IPv6AddressCount
=== RUN   TestAccEC2Instance_IPv6AddressesExplicit
=== PAUSE TestAccEC2Instance_IPv6AddressesExplicit
=== CONT  TestAccEC2Instance_IPv6_supportAddressCount
=== CONT  TestAccEC2Instance_IPv6AddressesExplicit
=== CONT  TestAccEC2Instance_IPv6_primaryEnable
=== CONT  TestAccEC2Instance_IPv6_supportAddressCountWithIPv4
=== CONT  TestAccEC2Instance_IPv6_primaryDisable
=== CONT  TestAccEC2Instance_IPv6AddressCountAndSingleAddressCausesError
=== CONT  TestAccEC2Instance_IPv6AddressCount
--- PASS: TestAccEC2Instance_IPv6AddressCountAndSingleAddressCausesError (2.23s)
--- PASS: TestAccEC2Instance_IPv6_supportAddressCountWithIPv4 (111.06s)
--- PASS: TestAccEC2Instance_IPv6_supportAddressCount (111.10s)
--- PASS: TestAccEC2Instance_IPv6_primaryEnable (147.70s)
--- PASS: TestAccEC2Instance_IPv6AddressCount (177.61s)
--- PASS: TestAccEC2Instance_IPv6_primaryDisable (177.66s)
--- PASS: TestAccEC2Instance_IPv6AddressesExplicit (354.97s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/ec2	361.486s

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