[Fleet] Fix output backfill for preconfigured outputs (#173088)

## Summary

Closes #173081 

Fixes output backfill for existing preconfigured outputs. 

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

---------

Co-authored-by: Nicolas Chaulet <nicolas.chaulet@elastic.co>

x-pack/plugins/fleet/common/services/output_helpers.test.ts

@@ -101,4 +101,20 @@ describe('outputYmlIncludesReservedPerformanceKey', () => {
       expect(outputYmlIncludesReservedPerformanceKey(configYml, safeLoad)).toBe(false);
     });
   });
+
+  describe('comment', () => {
+    it('returns false when reserved key is present only in a comment', () => {
+      const configYml = `true`;
+
+      expect(outputYmlIncludesReservedPerformanceKey(configYml, safeLoad)).toBe(false);
+    });
+  });
+
+  describe('empty string', () => {
+    it('returns false when YML is empty', () => {
+      const configYml = ``;
+
+      expect(outputYmlIncludesReservedPerformanceKey(configYml, safeLoad)).toBe(false);
+    });
+  });
 });

x-pack/plugins/fleet/common/services/output_helpers.ts

@@ -52,7 +52,10 @@ export function outputYmlIncludesReservedPerformanceKey(
   const parsedYml = safeLoad(configYml);
 
   if (!isObject(parsedYml)) {
-    return RESERVED_CONFIG_YML_KEYS.some((key) => parsedYml.includes(key));
+    if (typeof parsedYml === 'string') {
+      return RESERVED_CONFIG_YML_KEYS.some((key) => parsedYml.includes(key));
+    }
+    return false;
   }
 
   const flattenedYml = isObject(parsedYml) ? getFlattenedObject(parsedYml) : {};

x-pack/plugins/fleet/server/services/output.test.ts

@@ -1841,4 +1841,66 @@ describe('Output Service', () => {
       });
     });
   });
+
+  describe('backfillAllOutputPresets', () => {
+    it('should update non-preconfigured output', async () => {
+      const soClient = getMockedSoClient({});
+
+      soClient.find.mockResolvedValue({
+        saved_objects: [
+          {
+            ...mockOutputSO('non-preconfigured-output', {
+              is_preconfigured: false,
+              type: 'elasticsearch',
+            }),
+            score: 0,
+          },
+        ],
+        total: 1,
+        per_page: 1,
+        page: 1,
+      });
+
+      soClient.get.mockResolvedValue({
+        ...mockOutputSO('non-preconfigured-output', {
+          is_preconfigured: false,
+          type: 'elasticsearch',
+        }),
+      });
+
+      const promise = outputService.backfillAllOutputPresets(soClient, esClientMock);
+
+      await expect(promise).resolves.not.toThrow();
+    });
+
+    it('should update preconfigured output', async () => {
+      const soClient = getMockedSoClient({});
+
+      soClient.find.mockResolvedValue({
+        saved_objects: [
+          {
+            ...mockOutputSO('preconfigured-output', {
+              is_preconfigured: true,
+              type: 'elasticsearch',
+            }),
+            score: 0,
+          },
+        ],
+        total: 1,
+        per_page: 1,
+        page: 1,
+      });
+
+      soClient.get.mockResolvedValue({
+        ...mockOutputSO('preconfigured-output', {
+          is_preconfigured: true,
+          type: 'elasticsearch',
+        }),
+      });
+
+      const promise = outputService.backfillAllOutputPresets(soClient, esClientMock);
+
+      await expect(promise).resolves.not.toThrow();
+    });
+  });
 });

x-pack/plugins/fleet/server/services/output.ts

@@ -1023,7 +1023,13 @@ class OutputService {
       async (output) => {
         const preset = getDefaultPresetForEsOutput(output.config_yaml ?? '', safeLoad);
 
-        await outputService.update(soClient, esClient, output.id, { preset });
+        await outputService.update(
+          soClient,
+          esClient,
+          output.id,
+          { preset },
+          { fromPreconfiguration: true }
+        );
         await agentPolicyService.bumpAllAgentPoliciesForOutput(soClient, esClient, output.id);
       },
       {