fix(proxmox): handle AttributeError when Qemu Agent is not available

[puretension]

What does this PR do?

Fixes AttributeError when Proxmox VMs don't have Qemu Agent installed or available.

Motivation

Fixes #21300 - When Qemu Agent is not installed on Proxmox VMs, the /agent/get-host-name endpoint fails and causes an AttributeError. This breaks hostname collection for VMs without the agent.

Description of the changes

• Add AttributeError to the existing exception handling in _get_vm_hostname()
• Falls back to using vm_name when Qemu Agent is unavailable
• Maintains existing logging behavior for debugging

How to test the changes?

Test with Proxmox VMs that don't have Qemu Agent installed - the integration should continue working and use VM names as hostnames.

Review checklist

• Feature or bugfix MUST have appropriate tests (existing tests cover the fallback behavior)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA
• If you need to backport this PR to another branch, you can add the backport/ label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.01%. Comparing base (7b59ac0) to head (5d4b286).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

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[github-actions]

⚠️ The qa/skip-qa label has been added with shippable changes

The following files, which will be shipped with the agent, were modified in this PR and
the qa/skip-qa label has been added.

You can ignore this if you are sure the changes in this PR do not require QA. Otherwise, consider removing the label.

List of modified files that will be shipped with the agent

proxmox/changelog.d/21399.fixed
proxmox/datadog_checks/proxmox/check.py