[bugfix] aws_sagemaker_domain: Fix update failure when app_network_access_type is VpcOnly

[tabito-hara]

Rollback Plan

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

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

#45528 reported that updating the resource with app_network_access_type = "VpcOnly" results in the following error:

ValidationException: VpcId and Subnet IDs are required for updating domains to VpcOnly mode.

This PR fixes the issue by always including VpcId and SubnetIds in the update API request parameters when app_network_access_type = "VpcOnly".

A test case is added to the existing acceptance test where app_network_access_type is VpcOnly.

Relations

Closes #45528

Output from Acceptance Testing

$ make testacc TESTS='TestAccSageMaker_serial/Domain/(sharingSettings|domainSettings)' PKG=sagemaker
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
make: Running acceptance tests on branch: 🌿 b-aws_sagemaker_domain-fix_update_VpcOnly 🌿...
TF_ACC=1 go1.24.11 test ./internal/service/sagemaker/... -v -count 1 -parallel 20 -run='TestAccSageMaker_serial/Domain/(sharingSettings|domainSettings)'  -timeout 360m -vet=off
2025/12/11 02:05:28 Creating Terraform AWS Provider (SDKv2-style)...
2025/12/11 02:05:28 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccSageMaker_serial
=== PAUSE TestAccSageMaker_serial
=== CONT  TestAccSageMaker_serial
=== RUN   TestAccSageMaker_serial/Domain
=== RUN   TestAccSageMaker_serial/Domain/domainSettings
=== RUN   TestAccSageMaker_serial/Domain/sharingSettings
=== RUN   TestAccSageMaker_serial/Domain/domainSettingsDockerSettingsUpdated
--- PASS: TestAccSageMaker_serial (659.00s)
    --- PASS: TestAccSageMaker_serial/Domain (659.00s)
        --- PASS: TestAccSageMaker_serial/Domain/domainSettings (220.18s)
        --- PASS: TestAccSageMaker_serial/Domain/sharingSettings (222.13s)
        --- PASS: TestAccSageMaker_serial/Domain/domainSettingsDockerSettingsUpdated (216.69s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/sagemaker  663.177s

[github-actions]

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