bug: fluent-operator breaks if a secret in a namespace is not available for the output

### Describe the issue

Hello,

We have outputs in multiple namespaces, and in one of those namespaces, the secret defined in the output wasn't available. This has caused fluent-bit daemon sets to go into CrashLoopBackOff and to not properly start.

There were multiple errors in the operator log where it cannot find the secrets in all namespaces, even though the secrets were present in all of them, except one e.g. test-01:

```
2025-10-14T15:09:27Z  ERROR  Reconciler error  {"controller": "fluentbit", "controllerGroup": "fluentbit.fluent.io", "controllerKind": "FluentBit", "FluentBit": {"name":"fluent-bit-config","namespace":"test-01"}, "namespace": test-01", "name": "fluent-bit-config", "reconcileID": "516abca7-9351-4f88-8b30-ff54fadf9c16", "error": "Secret \"elasticsearch-credentials\" not found"}
2025-10-14T15:09:27Z  ERROR  Reconciler error  {"controller": "fluentbit", "controllerGroup": "fluentbit.fluent.io", "controllerKind": "FluentBit", "FluentBit": {"name":"fluent-bit-config","namespace":"test-02"}, "namespace": "test-02", "name": "fluent-bit-config", "reconcileID": "4feafd59-972e-444d-a861-b36185091492", "error": "Secret \"elasticsearch-credentials\" not found"}
2025-10-14T15:09:27Z  ERROR  Reconciler error  {"controller": "fluentbit", "controllerGroup": "fluentbit.fluent.io", "controllerKind": "FluentBit", "FluentBit": {"name":"fluent-bit-config","namespace":"test-03"}, "namespace": "test-03", "name": "fluent-bit-config", "reconcileID": "850d08bf-a5d8-4109-afc6-1bd6dcd2bf07", "error": "Secret \"elasticsearch-credentials\" not found"}
```

Re-creating the missing secret in the namespace test-01 fixed the issue.

Thanks,
Marjan

### To Reproduce

- Create one output per namespace (e.g. more than 3)
- Create the secret in each namespace that is used by the output
- Watch logs being collected
- Delete a secret from one of the namespaces
- Check the logs in the fluent-operator
- Restart one of the fluent-bit daemon sets

### Expected behavior

When a secret for the output is not available, I expect the fluent-operator to report it as an error, but not to crash the fluent-bit instances.

### Your Environment

```markdown
- Fluent Operator version: 3.4.0
- Container Runtime: containerd
- Operating system:
- Kernel version:
```

### How did you install fluent operator?

Via Helm.

### Additional context

_No response_

Reproduce the issue by creating outputs and secrets across namespaces, deleting one secret, and restarting a Fluent Bit DaemonSet while watching fluent-operator logs. Trace the FluentBit reconciliation path that reports the missing secret; done means the operator reports the missing secret as an error without causing Fluent Bit instances to crash.

Written by the indexing model from the issue text.