Add a way to opt out of the default pending component

Problem

Routes that render their own loading state have no way to opt out of the router's default pending component.

pendingComponent currently falls back to the configured default whenever a route is pending. Setting the option to null or undefined does not disable that fallback.

Proposed solution

Add an explicit opt-out value:

export const Route = createFileRoute('/example')({
  pendingComponent: 'never',
})

This should prevent the default pending component from rendering while preserving the route's pending behavior.

Opening this issue because there was PR #2658 addressing it but it is outdated now.

Start by tracing the pendingComponent option from createFileRoute and the router's default pending-component selection. Review the outdated PR #2658 for prior context, then locate the relevant pending-state tests or add coverage nearby. Done means accepting pendingComponent: 'never' so the default does not render while route pending behavior remains intact.