openssh_keypair: make fingerprint result a string (#57295)

The extant documentation says that the fingerprint return value is a
single string, but it is currently being returned as a split list.
Convert the returned value to a string as documented, and add some
basic test-case coverage for the return values.

test/integration/targets/openssh_keypair/tasks/main.yml

@@ -1,6 +1,7 @@
 - name: Generate privatekey1 - standard
   openssh_keypair:
     path: '{{ output_dir }}/privatekey1'
+  register: privatekey1_result
 
 - name: Generate privatekey2 - size 2048
   openssh_keypair:

test/integration/targets/openssh_keypair/tests/validate.yml

@@ -1,3 +1,33 @@
+- name: Log privatekey1 return values
+  debug:
+    var: privatekey1_result
+
+- name: Validate privatekey1 return fingerprint
+  assert:
+    that:
+      - privatekey1_result["fingerprint"] is string
+      - privatekey1_result["fingerprint"].startswith("SHA256:")
+  # only distro old enough that it still gives md5 with no prefix
+  when: ansible_distribution != 'CentOS' and ansible_distribution_major_version != '6'
+
+- name: Validate privatekey1 return public_key
+  assert:
+    that:
+      - privatekey1_result["public_key"] is string
+      - privatekey1_result["public_key"].startswith("ssh-rsa ")
+
+- name: Validate privatekey1 return size value
+  assert:
+    that:
+      - privatekey1_result["size"]|type_debug == 'int'
+      - privatekey1_result["size"] == 4096
+
+- name: Validate privatekey1 return key type
+  assert:
+    that:
+      - privatekey1_result["type"] is string
+      - privatekey1_result["type"] == "rsa"
+
 - name: Validate privatekey1 (test - RSA key with size 4096 bits)
   shell: "ssh-keygen -lf {{ output_dir }}/privatekey1 | grep -o -E '^[0-9]+'"
   register: privatekey1