`AgentPod` misses serviceaccounts of plugin in controller namespace

### Pre-requisites

- [X] I have double-checked my configuration
- [X] I can confirm the issue exists when I tested with `:latest`
- [X] I have searched existing issues and could not find a match for this bug
- [X] I'd like to contribute the fix myself (see [contributing guide](https://github.com/argoproj/argo-workflows/blob/main/docs/CONTRIBUTING.md))

### What happened/what did you expect to happen?

Related discussion: https://github.com/argoproj/argo-workflows/discussions/12566
Controller runs in namespace: `argo`, there is a plugin named `khaos-executor-plugin` and a serviceaccount named `khaos-executor-plugin`.
The sample workflow runs in namespace: `khaos-workflow`, there is a plugin named `hello-executor-plugin` and a serviceaccount named `hello-executor-plugin`.
```shell
argo get hello-7t78h -n khaos-workflow
Name: hello-7t78h
Namespace: khaos-workflow
ServiceAccount: unset (will run with the default ServiceAccount)
Status: Error
Message: serviceaccounts "khaos-executor-plugin" not found
Conditions:
Completed True
PodRunning False
Created: Wed Feb 28 17:32:52 +0800 (2 hours ago)
Started: Wed Feb 28 17:39:01 +0800 (2 hours ago)
Finished: Wed Feb 28 17:39:01 +0800 (2 hours ago)
Duration: 0 seconds
Progress: 0/1

STEP TEMPLATE PODNAME DURATION MESSAGE
◷ hello-7t78h main
```

### Version

v3.4.14

### Paste a small workflow that reproduces the issue. We must be able to run the workflow; don't enter a workflows that uses private images.

```YAML
apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
generateName: hello-
spec:
entrypoint: main
templates:
- name: main
plugin:
hello: { }
```

### Logs from the workflow controller

```text
time="2024-02-28T09:39:01.368Z" level=info msg="Task-result reconciliation" namespace=khaos-workflow numObjs=0 workflow=hello-7t78h
time="2024-02-28T09:39:01.368Z" level=warning msg="Node was nil, will be initialized as type Skipped" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.368Z" level=warning msg="[DEBUG] boundaryID was nil" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.368Z" level=info msg="was unable to obtain node for , letting display name to be nodeName" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.368Z" level=info msg="Plugin node hello-7t78h initialized Pending" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.368Z" level=info msg="TaskSet Reconciliation" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.368Z" level=info msg="Creating TaskSet" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.438Z" level=info msg=reconcileAgentPod namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.458Z" level=error msg="error in agent pod reconciliation" error="serviceaccounts \"khaos-executor-plugin\" not found" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.458Z" level=info msg="Updated phase Running -> Error" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.458Z" level=info msg="Updated message -> serviceaccounts \"khaos-executor-plugin\" not found" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.458Z" level=info msg="Marking workflow completed" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.458Z" level=info msg="Marking workflow as pending archiving" namespace=khaos-workflow workflow=hello-7t78h
time="2024-02-28T09:39:01.458Z" level=info msg="Workflow to be dehydrated" Workflow Size=1516
time="2024-02-28T09:39:01.466Z" level=info msg="cleaning up pod" action=deletePod key=khaos-workflow/hello-7t78h-1340600742-agent/deletePod
time="2024-02-28T09:39:01.468Z" level=info msg="Workflow update successful" namespace=khaos-workflow phase=Error resourceVersion=85399559051 workflow=hello-7t78h
time="2024-02-28T09:39:01.491Z" level=info msg="archiving workflow" namespace=khaos-workflow uid=5c4aa38d-4d94-4def-b189-b6908d3133e4 workflow=hello-7t78h
time="2024-02-28T09:39:01.513Z" level=info msg="Queueing Error workflow khaos-workflow/hello-7t78h for delete in 72h0m0s due to TTL"
time="2024-02-28T09:39:56.542Z" level=info msg="Alloc=17143 TotalAlloc=4920922 Sys=61565 NumGC=947 Goroutines=282"
time="2024-02-28T09:40:17.314Z" level=info msg="Performing periodic workflow GC"
time="2024-02-28T09:40:17.316Z" level=info msg="Deleting old offloads that are not live" len_wfs=0
time="2024-02-28T09:40:17.316Z" level=info msg="Workflow GC finished"
```

### Logs from in your workflow's wait container

```text
kubectl logs -n argo -c wait -l workflows.argoproj.io/workflow=${workflow},workflow.argoproj.io/phase!=Succeeded
```

Start by tracing the controller's reconcileAgentPod path using the provided workflow and controller logs, focusing on how the plugin serviceaccount is selected across the argo and khaos-workflow namespaces. Reproduce the AgentPod failure with the sample workflow, then verify that the expected plugin serviceaccount is found and the workflow no longer errors with a missing serviceaccount.

Written by the indexing model from the issue text.