Prepare before 1.15.9 release (#39043)

Co-authored-by: hc-github-team-tf-core <github-team-tf-core@hashicorp.com>

Author: hc-github-team-tf-core

.changes/1.15.9.md

@@ -0,0 +1,13 @@
+## 1.15.9 (August 19, 2026)
+
+
+BUG FIXES:
+
+* validate: Child module validation has been fixed and will now raise errors or warning diagnostics for invalid blocks. (`list`, `import`, `backend`, and `cloud`) ([#38994](https://github.com/hashicorp/terraform/issues/38994))
+
+
+NOTES:
+
+* Update go-slug to v0.18.3 to mitigate CVE-2026-14978, which is a Unicode normalization issue that could lead to files not being correctly excluded via `.terraformignore` from upload to a Terraform Enterprise or HCP Terraform during a run ([#39036](https://github.com/hashicorp/terraform/issues/39036))
+
+

.changes/v1.15/BUG FIXES-20260810-095651.yaml

@@ -1,4 +1,15 @@
-## 1.15.9 (Unreleased)
+## 1.15.9 (August 19, 2026)
+
+
+BUG FIXES:
+
+* validate: Child module validation has been fixed and will now raise errors or warning diagnostics for invalid blocks. (`list`, `import`, `backend`, and `cloud`) ([#38994](https://github.com/hashicorp/terraform/issues/38994))
+
+
+NOTES:
+
+* Update go-slug to v0.18.3 to mitigate CVE-2026-14978, which is a Unicode normalization issue that could lead to files not being correctly excluded via `.terraformignore` from upload to a Terraform Enterprise or HCP Terraform during a run ([#39036](https://github.com/hashicorp/terraform/issues/39036))
+
 
 ## 1.15.8 (July 8, 2026)
 
@@ -198,15 +209,7 @@ NOTES:
 UPGRADE NOTES:
 
 * backend/s3: The `AWS_USE_FIPS_ENDPOINT` and `AWS_USE_DUALSTACK_ENDPOINT` environment variables now only respect `true` or `false` values, aligning with the AWS SDK for Go. This replaces the previous behavior which treated any non-empty value as `true`. ([#37601](https://github.com/hashicorp/terraform/issues/37601))
-EXPERIMENTS:
-
-Experiments are only enabled in alpha releases of Terraform CLI. The following features are not yet available in stable releases.
 
-- The experimental "deferred actions" feature, enabled by passing the `-allow-deferral` option to `terraform plan`, permits `count` and `for_each` arguments in `module`, `resource`, and `data` blocks to have unknown values and allows providers to react more flexibly to unknown values.
-- `terraform test cleanup`: The experimental `test cleanup` command. In experimental builds of Terraform, a manifest file and state files for each failed cleanup operation during test operations are saved within the `.terraform` local directory. The `test cleanup` command will attempt to clean up the local state files left behind automatically, without requiring manual intervention.
-- `terraform test`: `backend` blocks and `skip_cleanup` attributes:
-  - Test authors can now specify `backend` blocks within `run` blocks in Terraform Test files. Run blocks with `backend` blocks will load state from the specified backend instead of starting from empty state on every execution. This allows test authors to keep long-running test infrastructure alive between test operations, saving time during regular test operations.
-  - Test authors can now specify `skip_cleanup` attributes within test files and within run blocks. The `skip_cleanup` attribute tells `terraform test` not to clean up state files produced by run blocks with this attribute set to true. The state files for affected run blocks will be written to disk within the `.terraform` directory, where they can then be cleaned up manually using the also experimental `terraform test cleanup` command.
 
 ## Previous Releases
 

.changes/v1.15/NOTES-20260818-182823.yaml

@@ -1 +1 @@
-1.15.9-dev
+1.15.9