feat: assert that the signing certificate authority is available

Issuing an intermediate certificate authority or a client certificate always
signs against the private key of the parent authority, regardless of whether
that authority is managed in the same run. With `certificate_authority_root_ca_skip`
or `certificate_authority_intermediate_ca_skip` enabled and no previously
provisioned key at the configured path, this surfaced as a generic module error
deep inside the signing task.

Stat the parent private key upfront and assert its presence, so the failure
names the missing path and the variables that control it. Skipping the parent
remains valid when its key already exists, because the check inspects the file
instead of the skip variable.

Co-authored-by: Copilot <copilot@github.com>

tasks/intermediate_certificate_authority.yaml

@@ -8,6 +8,23 @@
     mode: "0700"
     state: "directory"
 
+- name: Verify that the signing root Certificate Authority (CA) is available
+  when: certificate_authority_intermediate_ca_create is defined and
+        certificate_authority_intermediate_ca_create
+  block:
+  - name: Check private key of the root Certificate Authority (CA)
+    ansible.builtin.stat:
+      path: "{{ certificate_authority_root_ca_path }}/privkey.pem"
+    register: _root_ca_privkey
+  - name: Assert that the private key of the root Certificate Authority (CA) exists
+    ansible.builtin.assert:
+      that: _root_ca_privkey.stat.exists
+      fail_msg: >-
+        Signing the intermediate certificate authority requires
+        {{ certificate_authority_root_ca_path }}/privkey.pem. Either unset
+        certificate_authority_root_ca_skip so the root certificate authority is created or
+        imported, or point certificate_authority_root_ca_path to an existing one.
+
 - name: Create unprotected intermediate Certificate Authority (CA)
   ansible.builtin.include_tasks: intermediate_certificate_authority_unprotected.yaml
   when: certificate_authority_intermediate_ca_create is defined and