## Problem
While working on changes for bulk reassign https://github.com/elastic/kibana/issues/90437, I found that the server has a runtime error and returns a 500 if given an invalid or missing id.
<details><summary>server error stack trace</summary>
```
│ proc [kibana] server log [12:21:48.953] [error][fleet][plugins] TypeError: Cannot read property 'policy_revision_idx' of undefined
│ proc [kibana] at map (/Users/jfsiii/work/kibana/x-pack/plugins/fleet/server/services/agents/helpers.ts:15:34)
│ proc [kibana] at Array.map (<anonymous>)
│ proc [kibana] at getAgents (/Users/jfsiii/work/kibana/x-pack/plugins/fleet/server/services/agents/crud.ts:191:32)
│ proc [kibana] at runMicrotasks (<anonymous>)
│ proc [kibana] at processTicksAndRejections (internal/process/task_queues.js:93:5)
│ proc [kibana] at Object.reassignAgents (/Users/jfsiii/work/kibana/x-pack/plugins/fleet/server/services/agents/reassign.ts:91:9)
│ proc [kibana] at postBulkAgentsReassignHandler (/Users/jfsiii/work/kibana/x-pack/plugins/fleet/server/routes/agent/handlers.ts:314:21)
│ proc [kibana] at Router.handle (/Users/jfsiii/work/kibana/src/core/server/http/router/router.ts:272:30)
│ proc [kibana] at handler (/Users/jfsiii/work/kibana/src/core/server/http/router/router.ts:227:11)
│ proc [kibana] at exports.Manager.execute (/Users/jfsiii/work/kibana/node_modules/@hapi/hapi/lib/toolkit.js:60:28)
│ proc [kibana] at Object.internals.handler (/Users/jfsiii/work/kibana/node_modules/@hapi/hapi/lib/handler.js:46:20)
│ proc [kibana] at exports.execute (/Users/jfsiii/work/kibana/node_modules/@hapi/hapi/lib/handler.js:31:20)
│ proc [kibana] at Request._lifecycle (/Users/jfsiii/work/kibana/node_modules/@hapi/hapi/lib/request.js:370:32)
│ proc [kibana] at Request._execute (/Users/jfsiii/work/kibana/node_modules/@hapi/hapi/lib/request.js:279:9)
```
</details>
<details><summary>see test added in this PR fail on master</summary>
```
1) Fleet Endpoints
reassign agent(s)
bulk reassign agents
should allow to reassign multiple agents by id -- some invalid:
Error: expected 200 "OK", got 500 "Internal Server Error"
```
</details>
## Root cause
Debugging runtime error in `searchHitToAgent` found some TS type mismatches for the ES values being returned. Perhaps from one or more of the recent changes to ES client & Fleet Server. Based on `test:jest` and `test:ftr`, it appears the possible types are `GetResponse` or `SearchResponse`, instead of only an `ESSearchHit`.
https://github.com/elastic/kibana/pull/94632/files#diff-254d0f427979efc3b442f78762302eb28fb9c8857df68ea04f8d411e052f939cL11
While a `.search` result will include return matched values, a `.get` or `.mget` will return a row for each input and a `found: boolean`. e.g. `{ _id: "does-not-exist", found: false }`. The error occurs when [`searchHitToAgent`](1702cf98f0/x-pack/plugins/fleet/server/services/agents/helpers.ts (L11)) is run on a get miss instead of a search hit.
## PR Changes
* Added a test to ensure it doesn't fail if invalid or missing IDs are given
* Moved the `bulk_reassign` tests to their own test section
* Filter out any missing results before calling `searchHitToAgent`, to match current behavior
* Consolidate repeated arguments into and code for getting agents into single [function](https://github.com/elastic/kibana/pull/94632/files#diff-f7377ed9ad56eaa8ea188b64e957e771ccc7a7652fd1eaf44251c25b930f8448R70-R87): and [TS type](https://github.com/elastic/kibana/pull/94632/files#diff-f7377ed9ad56eaa8ea188b64e957e771ccc7a7652fd1eaf44251c25b930f8448R61-R68)
* Rename some agent service functions to be more explicit (IMO) but behavior maintained. Same API names exported.
This moves toward the "one result (success or error) per given id" approach for https://github.com/elastic/kibana/issues/90437
### 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