test(techdocs-node): sequence node custom tag validation failure

This test proves that currently only scalar YAML nodes are considered valid by
the mkdocs YAML validation logic, even though using the `!ENV` tag with
sequential data is officially supported by mkdocs[0].

[0] https://www.mkdocs.org/user-guide/configuration/#environment-variables

Signed-off-by: Viktor Bahr <viktor.bahr@tier.app>

plugins/techdocs-node/src/stages/generate/__fixtures__/mkdocs_with_env_tag.yml

@@ -0,0 +1 @@
+site_name: !ENV [SITE_NAME, OTHER_NAME, 'My default site name']

plugins/techdocs-node/src/stages/generate/helpers.test.ts

@@ -78,6 +78,9 @@ const mkdocsYmlWithoutPlugins = fs.readFileSync(
 const mkdocsYmlWithAdditionalPlugins = fs.readFileSync(
   resolvePath(__filename, '../__fixtures__/mkdocs_with_additional_plugins.yml'),
 );
+const mkdocsYmlWithEnvTag = fs.readFileSync(
+  resolvePath(__filename, '../__fixtures__/mkdocs_with_env_tag.yml'),
+);
 const mockLogger = getVoidLogger();
 const warn = jest.spyOn(mockLogger, 'warn');
 
@@ -582,10 +585,16 @@ describe('helpers', () => {
       ).rejects.toThrow();
     });
 
-    it('should validate files with custom yaml tags', async () => {
+    it('should validate files with custom yaml tags (scalar)', async () => {
       await expect(
         validateMkdocsYaml(inputDir, mkdocsYmlWithExtensions.toString()),
       ).resolves.toBeUndefined();
     });
+
+    it('should validate files with custom yaml tags (sequence)', async () => {
+      await expect(
+        validateMkdocsYaml(inputDir, mkdocsYmlWithEnvTag.toString()),
+      ).resolves.toBeUndefined();
+    });
   });
 });