Agent Logging Level Automatically Resets to Default i.e Info After Upgrading Agents on an Upgraded Kibana Build

- https://github.com/elastic/elastic-agent/issues/12949

**Kibana Build details:**
```
VERSION: 9.3.1 BC3
BUILD: 95740
COMMIT: 342266a63c5cedf139febf5a982ceb9a1c46690b
```
**Preconditions:**

1. Agents should be enrolled in 9.2.x.
2. The logging level of the agents should be set to either Debug, Warn, or Error on the 9.2.x build.
3. Kibana should be upgraded from 9.2.x to 9.3.1 BC3.

**Steps to Reproduce:**

1. Navigate to Assets > Agents tab.
2. Verify the logging level of the agent.
3. Navigate back to the Agents tab.
4. Click on the Action button of the same Agent.
5. Select Upgrade Agent and upgrade the agent to version 9.3.1 BC3.
6. Wait for the agent to upgrade.
7. Open Agent Details and observe the logging level of the agent has changed to Info, i.e., the default logging level.

**Expected Result:**

- Agent logging level should remain unchanged after upgrading the agent using an upgraded Kibana build.

**Screen Capture:**

https://github.com/user-attachments/assets/ada3610f-b3ac-4f34-8ee1-588541523413

**After Agent Upgrade**

**What's Working Fine:**

- The logging level does not change when the agent upgrade is performed without upgrading the build.

https://github.com/user-attachments/assets/53e68800-f11e-4b0f-873e-4a64f6c0da55

Start by tracing the Assets > Agents upgrade flow and the Agent Details logging-level state across a Kibana upgrade from 9.2.x to 9.3.1 BC3. Compare it with an agent-only upgrade, then verify that Debug, Warn, and Error remain unchanged after the upgraded-build flow completes.

Written by the indexing model from the issue text.