fix(monitoring-agents)!: isolate merged log payload under log_processed (#5167)

## What this PR does

Fluent Bit's `kubernetes` filter runs with `Merge_Log On` but without
`Merge_Log_Key`, so a parsed JSON log line is merged into the record
root.
An application that logs JSON can thereby set top-level fields that
later
pipeline stages trust for routing and tenant isolation:
`kubernetes_namespace_name`
(rewrite_tag routing and the `_stream_fields` stream identity),
`kubernetes.labels`, and the `tenant` / `cluster` / `log_source`
markers.
A tenant workload can forge another tenant's values on its own log
lines.

Setting `Merge_Log_Key log_processed` nests the parsed payload under a
dedicated key, so application log content can no longer override the
top-level service fields.

The change is applied to both Fluent Bit configurations that carry this
filter: the platform agent
(`packages/system/monitoring-agents/values.yaml`)
and the guest-cluster agent (`packages/apps/kubernetes`).

### Screenshots

Not a UI change.

### Downstream repositories

Walked the trigger map in `docs/agents/contributing.md` against the
diff,
file by file. The change touches only the Fluent Bit filter config
inside
`monitoring-agents`; it does not alter package layout, any
`values.schema.json`,
platform values keys, `ApplicationDefinition`, any CRD, `hack/`,
namespaces,
telemetry metric names, or the commit convention. None of the listed
couplings match.

- [x] No downstream repository is affected by this change

### Release note

```release-note
fix(monitoring-agents)!: Fluent Bit now nests parsed JSON log content under a `log_processed` key instead of merging it into the record root. This prevents an application log line from forging the service fields used for log routing and tenant isolation. BREAKING: top-level fields previously extracted from JSON logs (for example `level`, `msg`, and any application-specific keys) now live under `log_processed.*`; external dashboards and LogsQL queries that read them from the record root must be updated.
```


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

<!-- coderabbit-ai-generated:v1 -->

## Summary by CodeRabbit

* **Improvements**
* Merged Kubernetes log content is now grouped under `log_processed` in
Fluent Bit output.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/apps/kubernetes/templates/helmreleases/monitoring-agents.yaml

@@ -81,6 +81,7 @@ spec:
               Name kubernetes
               Match kube.*
               Merge_Log On
+              Merge_Log_Key log_processed
               Keep_Log On
               K8S-Logging.Parser On
               K8S-Logging.Exclude On

packages/system/monitoring-agents/values.yaml

@@ -450,6 +450,7 @@ fluent-bit:
           Name kubernetes
           Match kube.*
           Merge_Log On
+          Merge_Log_Key log_processed
           Keep_Log On
           K8S-Logging.Parser On
           K8S-Logging.Exclude On