[Enhancement] aws_docdbelastic_cluster: Add shard_instance_count argument

[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

This PR adds shard_instance_count argument for aws_docdbelastic_cluster resource.

• Even if aws_docdbelastic_cluster is not specified, the AWS API returns its default value 2. Therefore, shard_instance_count is marked as Computed.

Relations

Closes #46924

References

https://docs.aws.amazon.com/documentdb/latest/developerguide/API_elastic_CreateCluster.html#documentdb-elastic_CreateCluster-request-shardInstanceCount

https://docs.aws.amazon.com/documentdb/latest/developerguide/elastic-get-started.html#elastic-get-started-clusters

c. In the Shard instance count field, choose the number of replica instances you want associated with each shard. The maximum number of shard instances is 16, in increments of 1. All replica instances have the same shard capacity as defined in the following field. For testing purposes, the default value of 2 should suffice.

Output from Acceptance Testing

$ make testacc TESTS='TestAccDocDBElasticCluster_' PKG=docdbelastic 
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
make: Running acceptance tests on branch: 🌿 f-aws_docdbelastic_cluster-add_shard_instance_count 🌿...
TF_ACC=1 go1.25.8 test ./internal/service/docdbelastic/... -v -count 1 -parallel 20 -run='TestAccDocDBElasticCluster_'  -timeout 360m -vet=off
2026/03/14 23:56:48 Creating Terraform AWS Provider (SDKv2-style)...
2026/03/14 23:56:48 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccDocDBElasticCluster_Identity_basic
=== PAUSE TestAccDocDBElasticCluster_Identity_basic
=== RUN   TestAccDocDBElasticCluster_Identity_regionOverride
=== PAUSE TestAccDocDBElasticCluster_Identity_regionOverride
=== RUN   TestAccDocDBElasticCluster_Identity_ExistingResource_basic
=== PAUSE TestAccDocDBElasticCluster_Identity_ExistingResource_basic
=== RUN   TestAccDocDBElasticCluster_Identity_ExistingResource_noRefreshNoChange
=== PAUSE TestAccDocDBElasticCluster_Identity_ExistingResource_noRefreshNoChange
=== RUN   TestAccDocDBElasticCluster_basic
=== PAUSE TestAccDocDBElasticCluster_basic
=== RUN   TestAccDocDBElasticCluster_disappears
=== PAUSE TestAccDocDBElasticCluster_disappears
=== RUN   TestAccDocDBElasticCluster_tags
=== PAUSE TestAccDocDBElasticCluster_tags
=== RUN   TestAccDocDBElasticCluster_update
=== PAUSE TestAccDocDBElasticCluster_update
=== RUN   TestAccDocDBElasticCluster_shardInstanceCount
=== PAUSE TestAccDocDBElasticCluster_shardInstanceCount
=== CONT  TestAccDocDBElasticCluster_Identity_basic
=== CONT  TestAccDocDBElasticCluster_disappears
=== CONT  TestAccDocDBElasticCluster_update
=== CONT  TestAccDocDBElasticCluster_tags
=== CONT  TestAccDocDBElasticCluster_Identity_ExistingResource_basic
=== CONT  TestAccDocDBElasticCluster_Identity_regionOverride
=== CONT  TestAccDocDBElasticCluster_shardInstanceCount
=== CONT  TestAccDocDBElasticCluster_Identity_ExistingResource_noRefreshNoChange
=== CONT  TestAccDocDBElasticCluster_basic
--- PASS: TestAccDocDBElasticCluster_disappears (1806.35s)
--- PASS: TestAccDocDBElasticCluster_Identity_basic (1924.01s)
--- PASS: TestAccDocDBElasticCluster_tags (1985.47s)
--- PASS: TestAccDocDBElasticCluster_Identity_ExistingResource_basic (2057.37s)
--- PASS: TestAccDocDBElasticCluster_shardInstanceCount (2067.40s)
--- PASS: TestAccDocDBElasticCluster_Identity_ExistingResource_noRefreshNoChange (2121.86s)
--- PASS: TestAccDocDBElasticCluster_Identity_regionOverride (2137.44s)
--- PASS: TestAccDocDBElasticCluster_basic (2139.44s)
--- PASS: TestAccDocDBElasticCluster_update (2376.41s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/docdbelastic       2380.808s

[github-actions]

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