Description:

This PR fixes an issue where multiple concurrent useFetcher() submissions would collapse into a single effective inflight fetcher on routes that use deferred loaders with Suspense/Await.

Problem
On deferred routes, only the first fetcher would enter the loading state while others remained in submitting state or vanished entirely, breaking parallel optimistic rendering. This was different from blocking routes where multiple concurrent fetchers worked correctly.

Solution
Modified the getUpdatedRevalidatingFetchers function in packages/react-router/lib/router/router.ts to preserve fetchers that are already in "submitting" or "loading" states during deferred revalidation. The fix adds a condition to only update fetchers that are not already active:

 if (!fetcher || (fetcher.state !== "submitting" && fetcher.state !== "loading")) {
    // Update fetcher state
 }

Changes Made
1. Updated getUpdatedRevalidatingFetchers function to preserve active fetchers during revalidation
2. Added proper test cases to verify the fix works correctly
3. Created changeset file to document the fix

Verification
- Multiple concurrent fetchers now maintain independent states on deferred routes
- Behavior is now consistent between blocking and deferred routes
- All fetchers progress independently: submitting → loading → idle
- Maintains backward compatibility with existing functionality