[Fleet] add kibana version if air gapped or product versions doesn't return (#174324)

## Summary

Closes https://github.com/elastic/ingest-dev/issues/2779

Adding the kibana version to the list of available agent versions if the
product versions api doesn't return any versions (not accessible or
airgapped environment). This is a best effort fix to add the missing
latest released version if the build version list is outdated.

To verify:
1. test with internet connection
- enroll an agent with the latest version 8.11.3 locally or in a VM (not
container)
- check that the upgrade available badge is not showing up and the add
agent instructions have 8.11.3 version

<img width="1324" alt="image"
src="b833c92e-3864-4a0b-a26b-0cf927b95a80">
<img width="814" alt="image"
src="bb21a63a-dbd5-4557-b81d-93391bbc51a9">


Restart kibana or wait 10m for the cache to expire before testing with
air gapped.

2. test air-gapped by adding `xpack.fleet.isAirGapped: true` in
kibana.yml
- test that the agent shows up with the badge upgrade available, and the
upgrade agent modal has 8.13 version locally
- check that the add agent instructions have the 8.13 version (current
kibana version)

<img width="1294" alt="image"
src="81df38c4-cc35-466b-9d41-c226b8b29563">
<img width="818" alt="image"
src="0a1b72d8-be3c-4cbf-9f44-30a7f7aef02a">




### 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

x-pack/plugins/fleet/server/services/agents/versions.test.ts

@@ -137,7 +137,7 @@ describe('getAvailableVersions', () => {
   });
 
   it('should cache results', async () => {
-    mockKibanaVersion = '300.0.0';
+    mockKibanaVersion = '9.0.0';
     mockedReadFile.mockResolvedValue(`["8.1.0", "8.0.0", "7.17.0", "7.16.0"]`);
     mockedFetch.mockResolvedValueOnce({
       status: 200,
@@ -195,7 +195,7 @@ describe('getAvailableVersions', () => {
     const res = await getAvailableVersions({ ignoreCache: true });
 
     // Should sort, uniquify and filter out versions < 7.17
-    expect(res).toEqual(['8.1.0', '8.0.0', '7.17.0']);
+    expect(res).toEqual(['300.0.0', '8.1.0', '8.0.0', '7.17.0']);
   });
 
   it('should gracefully handle network errors when fetching from product versions API', async () => {
@@ -206,7 +206,7 @@ describe('getAvailableVersions', () => {
     const res = await getAvailableVersions({ ignoreCache: true });
 
     // Should sort, uniquify and filter out versions < 7.17
-    expect(res).toEqual(['8.1.0', '8.0.0', '7.17.0']);
+    expect(res).toEqual(['300.0.0', '8.1.0', '8.0.0', '7.17.0']);
   });
 
   it('should not fetch from product versions API when air-gapped config is set', async () => {
@@ -216,7 +216,7 @@ describe('getAvailableVersions', () => {
     mockConfig = { isAirGapped: true };
     const res = await getAvailableVersions({ ignoreCache: true });
 
-    expect(res).toEqual(['8.1.0', '8.0.0', '7.17.0']);
+    expect(res).toEqual(['300.0.0', '8.1.0', '8.0.0', '7.17.0']);
     expect(mockedFetch).not.toBeCalled();
   });
 });

x-pack/plugins/fleet/server/services/agents/versions.ts

@@ -88,11 +88,12 @@ export const getAvailableVersions = async ({
     .filter((v: any) => semverGte(v, MINIMUM_SUPPORTED_VERSION))
     .sort((a: any, b: any) => (semverGt(a, b) ? -1 : 1));
 
-  // If the current stack version isn't included in the list of available versions, add it
-  // at the front of the array
-  const hasCurrentVersion = availableVersions.some((v) => v === kibanaVersion);
-  if (includeCurrentVersion && !hasCurrentVersion) {
-    availableVersions = [kibanaVersion, ...availableVersions];
+  // if api versions are empty (air gapped or API not available), we add current kibana version, as the build file might not contain the latest released version
+  if (
+    includeCurrentVersion ||
+    (apiVersions.length === 0 && !config?.internal?.onlyAllowAgentUpgradeToKnownVersions)
+  ) {
+    availableVersions = uniq([kibanaVersion, ...availableVersions]);
   }
 
   // Allow upgrading to the current stack version if this override flag is provided via `kibana.yml`.