[OpAMP] fleet-server ignores `AgentDescription` updates after enrollment

- Version: main
- Operating System: N/A
- Discuss Forum URL: N/A
- Steps to Reproduce:

## Summary

When an OpAMP client (e.g. OTel Collector) sends an `AgentToServer` message
after initial enrollment with an updated `AgentDescription` sub-message,
fleet-server silently ignores it. The stored agent document keeps whatever
`IdentifyingAttributes` / `NonIdentifyingAttributes` / hostname / version
were captured at enrollment time.

## Why this is a bug

The OpAMP specification explicitly allows `AgentDescription` to change, and
defines a compression rule where the client omits unchanged sub-messages:

> "The Client MAY compress the AgentToServer message by omitting the
> sub-messages that have not changed since that particular data was reported
> last time."

> "This field SHOULD be unset if this information is unchanged since the last
> AgentToServer message."

In other words, when `AgentDescription` *is* present on a non-initial message,
it means the client is signaling a change, and the server is expected to
update its view of the agent.

Today, `internal/pkg/api/handleOpAMP.go:updateAgent()` does not look at
`aToS.AgentDescription` at all — only `enrollAgent()` does. As a result:

- Hostname / OS / agent version changes are never reflected.
- `NonIdentifyingAttributes` and `IdentifyingAttributes` in `.fleet-agents`
remain frozen at enroll-time values.
- Any top-level fields derived from these attributes (e.g. tags promoted from
a non-identifying attribute) will also go stale as soon as a user edits
their collector config.

## Steps to Reproduce

1. Enroll an OTel Collector against fleet-server via OpAMP with
`non_identifying_attributes` containing `host.name: "host-a"`.
2. Confirm the agent document in `.fleet-agents` has `host.hostname: "host-a"`
in its local metadata, and that `host.name` appears in the stored
`NonIdentifyingAttributes`.
3. Stop the collector, change the config to `host.name: "host-b"`, restart it.
4. Observe that subsequent `AgentToServer` messages include the updated
`AgentDescription`, but the `.fleet-agents` document still shows
`host.hostname: "host-a"` and the old `NonIdentifyingAttributes` blob.

## Proposed fix

Extract the `AgentDescription` parsing logic from `enrollAgent()` into a
helper, and call it from `updateAgent()` as well when
`aToS.AgentDescription != nil`. Update the relevant fields on the existing
agent document (local metadata, identifying/non-identifying attributes,
and any derived top-level fields) via the bulker.

## Related

- [OpAMP specification — AgentToServer compression](https://github.com/open-telemetry/opamp-spec/blob/main/specification.md)