Fix proxy download test

[michel-laterman]

What does this PR do?

Fixes flaky TestFleetDownloadProxyURL test by changing how agent status is gathered.

Related issues

• Closes [Flaky Test]: TestFleetDownloadProxyURL – Unable to verify that upgrade has failed. #11796

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label that automatically backports to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[michel-laterman]

The agent control server uses a non-standard format when serializing timestamps breaking the ExecStatus command. I've added this method in order to avoid this.

[cmacknz]

This is weird, how is something formatted with time.Format not a valid time.Time?

Do you have an example of the error this causes?

I don't like just duplicating the function like this, it would be better to fix things in place, but I can't suggest alternatives unless I know what specific error we are working around.

[michel-laterman]

agent status returned an error: could not unmarshal agent status output: parsing time "2025-12-18 19:02:32 +0000 UTC" as "2006-01-02T15:04:05Z07:00": cannot parse " 19:02:32 +0000 UTC" as "T".

By default Go will try to to parse strings using the RFC3339 nano format (docs), the control server does not use this when serializing times to send to the client

[cmacknz]

Is there a drawback to changing to RFC3339? It seems like this would always unconditionally fail without a custom unmarshal implementation. Is this just the first time anyone has ever tried to unmarshal this as JSON in Go?

This is clearly just an artifact of the control server, the upgrade watcher is using gRPC instead so doesn't hit this I suspect because it never unmarshals as JSON.

Why do we have this non-standard format?

[michel-laterman]

I can't think of a drawback from a technical standpoint; i've made a draft to change it here:#11923.
I don't know why the non-standard format was chosen, perhaps @blakerouse or @michalpristas would know?