Type RxJS Next error values as unknown instead of any #7649

Follow-up to Discussion #7625.

RxJS Next public APIs currently expose several error values as any. That lets an error flow escape type checking even though JavaScript can throw any value and TypeScript treats catch bindings as unknown under strict settings.

Update the intentional RxJS Next error surface to use unknown consistently: observer/subscriber error callbacks, ColdObservable error delivery, subject error pathways, catchError, tap positional and observer-object forms, retry, notification error values, and the relevant platform catch/inspection callback types. Preserve the actual value at runtime.

This is a public type-only breaking change. Add focused strict type regressions that require narrowing before property access, and document the intentional compatibility break.