Fix unenroll inactive agent tasks if first set of agents returned is equal to UNENROLLMENT_BATCH_SIZE (#216283)

## Summary

Fix the unenrollment inactive agent task to be able to unenroll some
agents when the number of agents returned by the first
`getAgentsByKuery` trigger is equal to the limit of
`UNENROLLMENT_BATCH_SIZE`.


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---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

x-pack/platform/plugins/shared/fleet/server/tasks/unenroll_inactive_agents_task.test.ts

@@ -57,6 +57,7 @@ describe('UnenrollInactiveAgentsTask', () => {
   let mockTaskManagerSetup: jest.Mocked<TaskManagerSetupContract>;
   const mockedUnenrollBatch = jest.mocked(unenrollBatch);
 
+  const unenrollBatchSize = 3;
   const agents = [
     {
       id: 'agent-1',
@@ -91,6 +92,7 @@ describe('UnenrollInactiveAgentsTask', () => {
       core: mockCore,
       taskManager: mockTaskManagerSetup,
       logFactory: loggingSystemMock.create(),
+      unenrollBatchSize,
     });
   });
 

x-pack/platform/plugins/shared/fleet/server/tasks/unenroll_inactive_agents_task.ts

@@ -40,6 +40,7 @@ interface UnenrollInactiveAgentsTaskSetupContract {
   core: CoreSetup;
   taskManager: TaskManagerSetupContract;
   logFactory: LoggerFactory;
+  unenrollBatchSize?: number;
 }
 
 interface UnenrollInactiveAgentsTaskStartContract {
@@ -50,10 +51,13 @@ export class UnenrollInactiveAgentsTask {
   private logger: Logger;
   private wasStarted: boolean = false;
   private abortController = new AbortController();
+  private unenrollBatchSize: number;
 
   constructor(setupContract: UnenrollInactiveAgentsTaskSetupContract) {
-    const { core, taskManager, logFactory } = setupContract;
+    const { core, taskManager, logFactory, unenrollBatchSize } = setupContract;
     this.logger = logFactory.get(this.taskId);
+    this.unenrollBatchSize =
+      unenrollBatchSize !== undefined ? unenrollBatchSize : UNENROLLMENT_BATCHSIZE;
 
     taskManager.registerTaskDefinitions({
       [TYPE]: {
@@ -140,7 +144,7 @@ export class UnenrollInactiveAgentsTask {
         kuery,
         showInactive: true,
         page: 1,
-        perPage: UNENROLLMENT_BATCHSIZE,
+        perPage: this.unenrollBatchSize,
       });
       if (!res.agents.length) {
         this.logger.debug(
@@ -149,7 +153,7 @@ export class UnenrollInactiveAgentsTask {
         continue;
       }
       agentCounter += res.agents.length;
-      if (agentCounter >= UNENROLLMENT_BATCHSIZE) {
+      if (agentCounter > this.unenrollBatchSize) {
         this.endRun('Reached the maximum amount of agents to unenroll, exiting.');
         return;
       }