[Response Ops][Task Manager] Changing task manager schedule.interval schema to string from duration (#204413)

## Summary

Currently, when task manager schema changes are migrated down (which can
occur during a release when there are multiple Kibana nodes running and
they get updated one after another), we are running into this bug:
https://github.com/elastic/kibana/issues/204395 where the task
`schedule.interval` gets mutated from expected values (like `60s`) to
unallowed (but still valid for the schema) values (like `PT1M`).

This changes the schema for `schedule.interval` to use a string with
validation function.

## To verify
1. Run Kibana on `main` and create some rules that run frequently.
2. "Upgrade" to this PR branch and verify that rules continue to run.
3. "Downgrade" back to `main` and verify that rules continue to run.

x-pack/plugins/task_manager/server/saved_objects/model_versions/task_model_versions.ts

@@ -8,6 +8,10 @@
 import { SavedObjectsModelVersionMap } from '@kbn/core-saved-objects-server';
 import { taskSchemaV1, taskSchemaV2 } from '../schemas/task';
 
+// IMPORTANT!!!
+// When adding new model versions, make sure to manually test
+// downgrading to the previous version. This is a gap in our
+// automated test coverage so manual testing is needed.
 export const taskModelVersions: SavedObjectsModelVersionMap = {
   '1': {
     changes: [

x-pack/plugins/task_manager/server/saved_objects/schemas/task.test.ts

@@ -0,0 +1,24 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import { validateDuration } from './task';
+
+test('allows valid duration', () => {
+  expect(validateDuration('1s')).toBeUndefined();
+  expect(validateDuration('45346s')).toBeUndefined();
+  expect(validateDuration('10m')).toBeUndefined();
+  expect(validateDuration('30000000h')).toBeUndefined();
+  expect(validateDuration('3245d')).toBeUndefined();
+});
+
+test('returns error message for invalid duration', () => {
+  expect(validateDuration('10x')).toBe('string is not a valid duration: 10x');
+  expect(validateDuration('PT1M')).toBe('string is not a valid duration: PT1M');
+  expect(validateDuration('foo')).toBe('string is not a valid duration: foo');
+  expect(validateDuration('1 minute')).toBe('string is not a valid duration: 1 minute');
+  expect(validateDuration('1hr')).toBe('string is not a valid duration: 1hr');
+});

x-pack/plugins/task_manager/server/saved_objects/schemas/task.ts

@@ -6,6 +6,13 @@
  */
 
 import { schema } from '@kbn/config-schema';
+import { isInterval } from '../../lib/intervals';
+
+export function validateDuration(duration: string) {
+  if (!isInterval(duration)) {
+    return 'string is not a valid duration: ' + duration;
+  }
+}
 
 export const taskSchemaV1 = schema.object({
   taskType: schema.string(),
@@ -15,7 +22,7 @@ export const taskSchemaV1 = schema.object({
   runAt: schema.string(),
   schedule: schema.maybe(
     schema.object({
-      interval: schema.duration(),
+      interval: schema.string({ validate: validateDuration }),
     })
   ),
   params: schema.string(),