[Index Management] Restrict dot-prefixed index patterns in template f…

…orm (#193196)

Closes #190251

## Summary

This PR adds validation that restricts creating a template with a
dot-prefixed index pattern.

<img width="1194" alt="Screenshot 2024-09-18 at 10 49 47"
src="https://github.com/user-attachments/assets/f24c3e29-7db0-46fc-97de-52d4654073de">

Note: I tried adding tests for this validation
[here](https://github.com/elastic/kibana/blob/6a3adf73dacaeda073674ac4a10e8a2597e67739/x-pack/plugins/index_management/__jest__/client_integration/index_template_wizard/template_create.test.tsx#L163),
but it didn't work because the index pattern field is mocked in the
tests and errors are not triggered from invalid values.

x-pack/plugins/index_management/public/application/components/template_form/template_form_schemas.tsx

@@ -129,6 +129,18 @@ export const schemas: Record<string, FormSchema> = {
           type: VALIDATION_TYPES.ARRAY_ITEM,
           isBlocking: false,
         },
+        {
+          validator: startsWithField({
+            char: '.',
+            message: i18n.translate(
+              'xpack.idxMgmt.templateValidation.indexPatternDotPrefixedError',
+              {
+                defaultMessage: 'Index patterns cannot match dot-prefixed indices.',
+              }
+            ),
+          }),
+          type: VALIDATION_TYPES.ARRAY_ITEM,
+        },
       ],
     },
     doCreateDataStream: {