Describe the bug

When using include in Terragrunt with shallow merge (the default), the terraform { include_in_copy / exclude_from_copy } lists are not merged correctly:

• If both parent and child define lists, the parent values override the child values, even though shallow merge semantics are supposed to let the child win.
• If the parent has nil lists but the child defines them, the child’s values are still dropped.

This makes it impossible to set include_in_copy / exclude_from_copy in child configs.

Deep merge works as expected (lists are concatenated, parent first then child). This issue is only about shallow merge.

Steps To Reproduce

Folder structure

repo-root/
├── shared/
│   └── parent.hcl             # parent config
└── app/
    └── terragrunt.hcl         # child config

Parent (shared/tf_provider_azapi.hcl)

terraform {
  source = "git::ssh://[email protected]/example/repo//modules/tf-module"
  # no include_in_copy or exclude_from_copy here
}

Child (app/terragrunt.hcl)

include "parent" {
  path = find_in_parent_folders("shared/parent.hcl")
  # default merge_strategy = "shallow"
}

terraform {
  exclude_from_copy = ["**/_*"]
}

Current Behavior (Bug)

The child’s exclude_from_copy is dropped.
The effective config is just:

terraform {
  source = "git::ssh://[email protected]/example/repo//modules/azapi"
}

Expected behavior

In shallow merge, the child’s values should override the parent’s if non-nil:

terraform {
  source            = "git::ssh://[email protected]/example/repo//modules/tf-module"
  exclude_from_copy = ["**/_*"]
}

Nice to haves

• Terminal output
• Screenshots

Versions

• Terragrunt version: (tested on v0.86.2)
• OpenTofu/Terraform version: (e.g. Tofu v1.10.5)
• Environment details: (e.g. macOS 15.6.1)

Additional context

• This issue only affects shallow merge. Deep merge already works correctly (covered by tests).
• Currently forces duplication of terraform.source in parent configs to make filters work, which breaks DRY usage.
• Inconsistent with documented shallow-merge behaviour where child overrides parent for simple attributes.