fix(azure/aks): make ACR attach plan-stable for greenfield applies

[jcastiarena]

What

Fix a greenfield single-apply failure in the Azure aks module. attached_acr_id_map gated on var.acr_id != null, whose condition is unknown at plan time when the ACR is created in the same run — leaving the for_each key set unknown and failing the plan with Invalid for_each argument.

Change

Add an optional attach_acr bool (default null) and gate on it, falling back to the legacy behaviour when unset:

attached_acr_id_map = (var.attach_acr != null ? var.attach_acr : var.acr_id != null) ? { acr = var.acr_id } : {}

Backward compatible

The default (null) reproduces the current behaviour exactly — no existing consumer changes:

attach_acr	acr_id	Result
null (default)	known, non-null	attach (as today)
null (default)	known, null / omitted	no attach (as today)
null (default)	unknown at plan	fails (same as today — no regression)
true	unknown at plan	attach, plan-stable (the greenfield fix)
false	any	no attach

Greenfield single-apply is fixed by opting in with attach_acr = true; setting false force-disables.

Verification

• pre-commit run on the changed files: OpenTofu fmt / validate / tofu test → Passed
• commitlint → Passed
• README regenerated with terraform-docs