[Foundation] Make BuildContext's cancellation handlers testable

Follow-up from [#451](https://github.com/Fallout-build/Fallout/pull/451) (FT-2, [#307](https://github.com/Fallout-build/Fallout/issues/307)).

## Problem

`BuildContext.RegisterCancellationHandler` / `UnregisterCancellationHandler` — the two methods `BuildManager.CancellationHandler` exists to delegate to — have no positive coverage. The only spec on the facade (`Cancellation_handler_facade_is_a_no_op_without_an_active_context`) exercises the null-context path.

They can't be tested as written: registered handlers only run from the `Console.CancelKeyPress` subscription, and `ConsoleCancelEventArgs` has no public constructor, so the event can't be raised from a spec.

## Outcome

A registered handler can be shown to run, and an unregistered one shown not to, without depending on a real Ctrl+C.

Needs a seam — e.g. an `internal void InvokeCancellationHandlers()` on `BuildContext` that the `CancelKeyPress` subscription calls, so specs can drive the same path. Worth settling before FT-4/5/6 move more per-run services onto the context.

## Acceptance criteria

- [ ] Registering through `BuildManager.CancellationHandler` with an active context runs the handler when cancellation is signalled.
- [ ] Unregistering stops it running.
- [ ] Handlers registered against one context don't run for another.
- [ ] The seam stays `internal` — no `InternalsVisibleTo` to non-test assemblies.