[elastic_agent] Fix OS type counts in Overview dashboard to use host.os.type - #18251

Proposed commit message

WHAT: Replace host.os.family with host.os.type in the "[Elastic Agent] Overview" dashboard.
Updated the Platform filter control, the Linux metric panel (which negates macOS and Windows to derive
a Linux count), the macOS metric panel, and the Windows metric panel. Also updated the "darwin" filter
value to "macos" to match the standardized host.os.type values defined by ECS.

WHY: host.os.family contains many variants (e.g. "debian", "redhat", "darwin") and categorizes
operating systems inconsistently, leading to inaccurate OS counts in the dashboard. host.os.type
provides only the standardized values (linux, macos, windows) which map cleanly to the three
OS buckets the dashboard displays.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

Author's Checklist

• Verify the Linux, macOS, and Windows agent counts in the Overview dashboard match expectations after the field change.

How to test this PR locally

1. Install the elastic_agent integration from this branch.
2. Open the "[Elastic Agent] Overview" dashboard in Kibana.
3. Confirm the Linux, macOS, and Windows agent counts are accurate and consistent with the agents enrolled in Fleet.
4. Use the Platform filter control and confirm it now shows linux, macos, windows as options instead of family variants like darwin, debian, etc.

Related issues

• Closes [elastic_agent]: Elastic Agent Overview Dashboard #15278

Screenshots