Validate user and role names instead of munging binding names

Rancher requires global role binding names to be RFC 1123 labels. The
per-user binding name lowercased the key and replaced only "/", so
usernames containing ".", "_" or "@" still produced an invalid name and
failed at apply, and hyphenated names could collide (foo-bar + baz vs
foo + bar-baz) into one object.

Add validation blocks on var.users requiring the username and every
referenced role name to be RFC 1123 labels, so non-compliant input fails
the plan with an actionable message. The binding name is then built from
the two parts directly. A precondition rejects the remaining hyphen
ambiguity at plan time rather than as a mid-apply conflict. Document the
constraint in the users schema section of the README. The group binding
path is unchanged.

README.md

@@ -50,6 +50,15 @@ global_role_bindings:       # optional
 The password is read from Vault **at plan time**, so seed the kv-v2 secret
 before adding the file, or the plan fails.
 
+The file name (the username) and every entry in `global_role_bindings` must be
+an RFC 1123 label — `^[a-z0-9]([a-z0-9-]*[a-z0-9])?$`, i.e. lowercase
+alphanumerics and `-`, starting and ending alphanumeric. No `.`, `_`, `@` or
+uppercase. Rancher names each binding `akuser-<username>-<role>` and rejects
+anything else, so non-compliant values fail the plan with an explicit error
+instead of the apply. Names that are individually valid but collapse to the same
+binding name (e.g. `foo-bar` + `baz` and `foo` + `bar-baz`) are rejected at plan
+time too.
+
 ### `config/roles/<name>.yaml`
 
 ```yaml

modules/rancher/main.tf

@@ -90,11 +90,24 @@ locals {
 resource "rancher2_global_role_binding" "user" {
   for_each = local.user_global_role_bindings
 
-  # Binding name must be an RFC 1123 label, so lowercase it and swap the key's
-  # "/" separator for "-".
-  name           = "akuser-${lower(replace(each.key, "/", "-"))}"
+  # Both parts are RFC 1123 labels already (var.users validates them), so the
+  # binding name needs no transformation.
+  name           = "akuser-${each.value.username}-${each.value.global_role_id}"
   global_role_id = each.value.global_role_id
   user_id        = rancher2_user.this[each.value.username].id
+
+  lifecycle {
+    # Hyphens are legal inside both parts, so distinct bindings can still flatten
+    # to the same name (foo-bar + baz vs foo + bar-baz). Catch that at plan time
+    # rather than as a duplicate-object conflict mid-apply.
+    precondition {
+      condition = length(distinct([
+        for binding in local.user_global_role_bindings :
+        "akuser-${binding.username}-${binding.global_role_id}"
+      ])) == length(local.user_global_role_bindings)
+      error_message = "Two user global role bindings collapse to the same Rancher binding name (akuser-<username>-<role>). Rename one of the users or roles involved."
+    }
+  }
 }
 
 # Custom global roles. Rules are optional: a role with none grants nothing on its

modules/rancher/variables.tf

@@ -51,6 +51,28 @@ variable "users" {
     global_role_bindings = optional(list(string), [])
   }))
   default = {}
+
+  # Rancher names each global role binding as an RFC 1123 label and rejects
+  # anything else at apply time. The binding name is built from the username and
+  # the role name verbatim, so both must already be compliant — fail the plan
+  # with a clear message instead of munging the input and hoping.
+  validation {
+    condition = alltrue([
+      for username in keys(var.users) :
+      can(regex("^[a-z0-9]([a-z0-9-]*[a-z0-9])?$", username))
+    ])
+    error_message = "Usernames must be RFC 1123 labels: lowercase alphanumerics and '-', starting and ending alphanumeric (no '.', '_', '@' or uppercase). Rename config/users/<username>.yaml to a compliant username."
+  }
+
+  validation {
+    condition = alltrue(flatten([
+      for user in values(var.users) : [
+        for role in user.global_role_bindings :
+        can(regex("^[a-z0-9]([a-z0-9-]*[a-z0-9])?$", role))
+      ]
+    ]))
+    error_message = "Values in global_role_bindings must be RFC 1123 labels: lowercase alphanumerics and '-', starting and ending alphanumeric."
+  }
 }
 
 # Custom Rancher global roles (config/roles/<name>.yaml). Map key is the role