Add transit_gateway_attachment_id to aws_dx_gateway_association

[drewtul]

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

Add the computed attribute transit_gateway_attachment_id to aws_dx_gateway_association where type is transitGateway to enable users to access this value directly without an additional datasource causing issues in dependencies between modified resources and datasources.

Relations

Closes #35009

References

Output from Acceptance Testing

% make t T='TestAccDirectConnectGatewayAssociation_' K=directconnect ACCTEST_PARALLELISM=4
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.24.5 test ./internal/service/directconnect/... -v -count 1 -parallel 4 -run='TestAccDirectConnectGatewayAssociation_'  -timeout 360m -vet=off
2025/07/18 10:04:06 Creating Terraform AWS Provider (SDKv2-style)...
2025/07/18 10:04:06 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccDirectConnectGatewayAssociation_v0StateUpgrade
=== PAUSE TestAccDirectConnectGatewayAssociation_v0StateUpgrade
=== RUN   TestAccDirectConnectGatewayAssociation_v1StateUpgrade
=== PAUSE TestAccDirectConnectGatewayAssociation_v1StateUpgrade
=== RUN   TestAccDirectConnectGatewayAssociation_basicVPNGatewaySingleAccount
=== PAUSE TestAccDirectConnectGatewayAssociation_basicVPNGatewaySingleAccount
=== RUN   TestAccDirectConnectGatewayAssociation_basicVPNGatewayCrossAccount
=== PAUSE TestAccDirectConnectGatewayAssociation_basicVPNGatewayCrossAccount
=== RUN   TestAccDirectConnectGatewayAssociation_basicTransitGatewaySingleAccount
=== PAUSE TestAccDirectConnectGatewayAssociation_basicTransitGatewaySingleAccount
=== RUN   TestAccDirectConnectGatewayAssociation_basicTransitGatewayCrossAccount
=== PAUSE TestAccDirectConnectGatewayAssociation_basicTransitGatewayCrossAccount
=== RUN   TestAccDirectConnectGatewayAssociation_multiVPNGatewaysSingleAccount
=== PAUSE TestAccDirectConnectGatewayAssociation_multiVPNGatewaysSingleAccount
=== RUN   TestAccDirectConnectGatewayAssociation_allowedPrefixesVPNGatewaySingleAccount
=== PAUSE TestAccDirectConnectGatewayAssociation_allowedPrefixesVPNGatewaySingleAccount
=== RUN   TestAccDirectConnectGatewayAssociation_allowedPrefixesVPNGatewayCrossAccount
=== PAUSE TestAccDirectConnectGatewayAssociation_allowedPrefixesVPNGatewayCrossAccount
=== RUN   TestAccDirectConnectGatewayAssociation_recreateProposal
=== PAUSE TestAccDirectConnectGatewayAssociation_recreateProposal
=== CONT  TestAccDirectConnectGatewayAssociation_v0StateUpgrade
=== CONT  TestAccDirectConnectGatewayAssociation_basicTransitGatewayCrossAccount
=== CONT  TestAccDirectConnectGatewayAssociation_basicVPNGatewayCrossAccount
=== CONT  TestAccDirectConnectGatewayAssociation_allowedPrefixesVPNGatewayCrossAccount
--- PASS: TestAccDirectConnectGatewayAssociation_basicTransitGatewayCrossAccount (1406.58s)
=== CONT  TestAccDirectConnectGatewayAssociation_recreateProposal
--- PASS: TestAccDirectConnectGatewayAssociation_v0StateUpgrade (2017.06s)
=== CONT  TestAccDirectConnectGatewayAssociation_allowedPrefixesVPNGatewaySingleAccount
--- PASS: TestAccDirectConnectGatewayAssociation_basicVPNGatewayCrossAccount (2018.53s)
=== CONT  TestAccDirectConnectGatewayAssociation_basicTransitGatewaySingleAccount
--- PASS: TestAccDirectConnectGatewayAssociation_allowedPrefixesVPNGatewayCrossAccount (2530.29s)
=== CONT  TestAccDirectConnectGatewayAssociation_multiVPNGatewaysSingleAccount
--- PASS: TestAccDirectConnectGatewayAssociation_recreateProposal (1976.85s)
=== CONT  TestAccDirectConnectGatewayAssociation_basicVPNGatewaySingleAccount
=== CONT  TestAccDirectConnectGatewayAssociation_v1StateUpgrade
--- PASS: TestAccDirectConnectGatewayAssociation_basicTransitGatewaySingleAccount (1377.46s)
--- PASS: TestAccDirectConnectGatewayAssociation_allowedPrefixesVPNGatewaySingleAccount (1682.20s)
--- PASS: TestAccDirectConnectGatewayAssociation_multiVPNGatewaysSingleAccount (1500.42s)
--- PASS: TestAccDirectConnectGatewayAssociation_basicVPNGatewaySingleAccount (1234.34s)
--- PASS: TestAccDirectConnectGatewayAssociation_v1StateUpgrade (1430.26s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/directconnect      4831.026s

[github-actions]

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[ewbankkit]

transit_gateway_attachment_id was briefly added (but never released) in 708a9e2 (#8528) to address an eventual consistency error on aws_ec2_transit_gateway delete. That was addressed in #8752.
This is a separate use case.