fix: prevent fetcher formData race condition in optimistic UI (#14506)

[SarthakRawat-1]

Description

Fixes a race condition where fetcher.formData becomes undefined before navigation loaderData is available, causing UI flicker in optimistic update patterns.

Problem

The issue occurred because fetchers transitioned to idle state in processLoaderData before the navigation's loaderData was committed in completeNavigation. This created a gap where both formData and updated loaderData were unavailable, causing the UI to briefly show stale data.

Solution

• Keep fetchers in loading state with formData preserved until navigation completes
• Transition fetchers to idle atomically with loaderData updates in completeNavigation
• Ensures no gap where optimistic UI falls back to stale data

Changes

• Modified processLoaderData to preserve fetcher loading state during navigation
• Updated completeNavigation to handle fetcher state transitions atomically

Testing

• UI shows true -> true -> true (no flicker). Previously would have shown true -> false -> true (flicker)
• Test passes with the fix applied

Fixes #14506

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