Changes

Fixes #11152.

useMutationState() in @tanstack/svelte-query only ever grew its returned
array. The cache-subscription callback updated the underlying $state array
with Object.assign(result, nextResult), but Array.prototype.length is
non-enumerable, so Object.assign never truncates result when the new
result set is shorter. Once a mutation stopped matching filters (settled,
was removed/GC'd, or a narrower filter was applied), its stale entry stayed
in the array forever.

Fix: replace the Object.assign with result.splice(0, result.length, ...nextResult),
which replaces the array's contents in place — including truncating it —
while preserving the $state array's reference identity (required by the rune).

Includes a regression test that starts two mutations matching
filters: { status: 'pending' }, lets both settle, and asserts the returned
array goes back to [] instead of keeping the two stale "pending" entries.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

Summary by CodeRabbit

• Bug Fixes

  • Fixed useMutationState so mutations are removed from filtered results when they no longer match the selected criteria.
  • Improved reactive updates to ensure mutation state results stay accurate as statuses change.

• Tests

  • Added coverage for removing mutations that no longer match a pending-status filter.