GitOps dry-run should validate platforms for labels

[allenhouchins]

Fleet versions 4.84.0

• Discovered:
• Reproduced:

Web browser and operating system: Any

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💥  Actual behavior

GitOps dry-run is not currently validating platform in labels. This leads to discovering any issues during the real run.

🛠️ Expected behavior

TODO

🧑‍💻  Steps to reproduce

These steps:

• Have been confirmed to consistently lead to reproduction in multiple Fleet instances.
• Describe the workflow that led to the error, but have not yet been reproduced in multiple Fleet instances.

1. In GitOps, create a label and apply it to linux as the platform
2. Notice dry-run completes but the real run fails
3. Change platform to ubuntu or centos and observe successful run

🕯️ More info (optional)

Ran into this on dogfood: [+] applying 68 labels (1 new and 67 updated)
Error: POST /api/latest/fleet/spec/labels received status 422 Unprocessable Entity: invalid platform: darwin,linux,windows (API time: 122ms)
Error: Process completed with exit code 1.