Policies: new policy Save modal opens with no platforms checked #58701

Fleet versions

• Discovered: main (d7dbd06)

Web browser and operating system: Chromium on macOS

💥 Actual behavior

When you save a new policy, the save modal opens with no platforms checked (macOS, Windows, Linux and ChromeOS all unchecked). Clicking Save shows "Select at least one platform", so every new policy needs its platforms picked by hand. On 4.95.0-rc and 4.94.1 the modal opens with the query's compatible platforms checked, and new reports still do.

🛠️ Expected behavior

The save modal opens with the compatible platforms checked, as before and as on new reports.

🧑‍💻 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. Policies → Add policy (global or any fleet).
2. Enter SELECT 1; and click Save.
3. The modal's Target shows all four platforms unchecked.

🕯️ More info (optional)

• Introduced by Fleet UI: Validate SQL on blur, align form validation on reports/policies/labels #57312 (fcb629a, for Fleet UI: Update labels, reports, and policies forms validation #57393). It removed the default in PolicyForm.tsx that set an empty selection to getCompatiblePlatforms() for new policies (fleet-v4.94.1:frontend/pages/policies/edit/components/PolicyForm/PolicyForm.tsx:527). New policies have no platform selector on the page, so the selection is always empty when the modal opens.
• New reports keep the default (EditQueryForm.tsx, handleSaveQuery calls setSelectedPlatforms(getCompatiblePlatforms())).