fix(router-core): model error component props as unknown

Problem

errorComponent receives the raw value thrown by route loading and rendering code. That value can be an Error, a primitive, or an arbitrary object.

ErrorComponentProps<TError> does not currently propagate TError through the framework ErrorRouteComponent types, so route error components cannot safely use a narrower error type.

Expected behavior

The public error prop should be typed as unknown. Error components should narrow the value before accessing error-specific properties.

A future typed-error API can be considered only if an explicit error type can propagate through route configuration and the React, Solid, and Vue error-component contracts.

Scope

• Change the baseline error-component prop type to unknown.
• Update React, Solid, and Vue error-component contracts.
• Add type tests that require narrowing before accessing error-specific properties.
• Preserve runtime coverage for non-Error values in client and SSR error boundaries.
• Update the error-component API documentation.

Related Work

• #4686 introduced the original report.
• #4691 introduced the generic type that does not propagate through framework route components.
• #4727 attempts to resolve the mismatch by narrowing the prop to Error.

Start with the React, Solid, and Vue ErrorRouteComponent contracts, then inspect the existing client and SSR error-boundary tests. Update the type tests so error-specific properties require narrowing, preserve coverage for non-Error values, and revise the error-component API documentation.