Remove flaky test skip for AI assistant chat complete endpoint (#200871)

## Summary

This pull request includes a change to the
`x-pack/test_serverless/api_integration/test_suites/observability/ai_assistant/tests/complete/complete.spec.ts`
file. The change re-enables a previously skipped test for the
`/internal/observability_ai_assistant/chat/complete` endpoint.

* Re-enabled the test for the
`/internal/observability_ai_assistant/chat/complete` endpoint by
removing the `.skip` method from the `describe` block.


### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [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
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] The PR description includes the appropriate Release Notes section,
and the correct `release_node:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

x-pack/test_serverless/api_integration/test_suites/observability/ai_assistant/tests/complete/complete.spec.ts

@@ -63,8 +63,7 @@ export default function ApiTest({ getService }: FtrProviderContext) {
     },
   ];
 
-  // FLAKY: https://github.com/elastic/kibana/issues/197175
-  describe.skip('/internal/observability_ai_assistant/chat/complete', function () {
+  describe('/internal/observability_ai_assistant/chat/complete', function () {
     // TODO: https://github.com/elastic/kibana/issues/192751
     this.tags(['skipMKI']);
     let proxy: LlmProxy;
@@ -186,7 +185,11 @@ export default function ApiTest({ getService }: FtrProviderContext) {
 
       const parsedEvents = decodeEvents(receivedChunks.join(''));
 
-      expect(parsedEvents.map((event) => event.type)).to.eql([
+      expect(
+        parsedEvents
+          .map((event) => event.type)
+          .filter((eventType) => eventType !== StreamingChatResponseEventType.BufferFlush)
+      ).to.eql([
         StreamingChatResponseEventType.MessageAdd,
         StreamingChatResponseEventType.MessageAdd,
         StreamingChatResponseEventType.ChatCompletionChunk,