fix: Load unbound resource previews in the variable dialog

[kof]

New resources can fail to show data in the New variable dialog because a page-plan refresh cancels their preview request before it finishes.

This change keeps explicitly requested previews alive until the dialog releases them. Changing inputs, changing resource type, or closing the dialog cancels obsolete work and prevents old data from showing. Unused page resources remain lazy; a preview still makes an on-demand request when Load data is clicked. The dialog stays usable while unrelated page resources are loading.

Follow-up to #6030.

Checklist

• Reproduce the cancellation with a regression test that fails before the fix.
• Keep dialog previews independent of page resource discovery, using the existing loader, cache, and request versions.
• Clear old previews on input changes and release requests on dialog close.
• Cover new Current date, Sitemap, HTTP, and existing unbound resources, plus repeated loads and stale responses.
• Keep tests for obsolete page-request cancellation and async resource dependencies passing.
• Test the unsaved dialog with an unrelated page request pending and a page-plan recalculation.
• Clear the old preview when switching resource types or starting an asynchronous body edit.
• Review documentation impact: no update needed; this restores the documented preview behavior.
• Confirm GitHub CI passes.

Verification

• The dialog loading, type-switch, and body-edit regression tests failed before their fixes and passed afterward.
• 82 focused tests and the settings-panel suite (588 tests) passed.
• Builder typecheck and repository lint passed.
• With preview retention temporarily disabled, all nine new loader regression cases failed as expected.