ControllerCompositeDisposable: safer post-dispose behavior

[imurashka]

Problem

During Dispose() method, the composite enumerates its internal list. If a client adds a disposable at that time, the list is mutated and we get:
InvalidOperationException: Collection was modified; enumeration operation may not execute.

Root cause

Add/AddRange methods continued to push into the internal list after disposal had begun.

Fix

If the composite is disposed, Add/AddRange dispose the incoming item(s) immediately.

Notes

• API unchanged; callers that add after disposal simply have their disposables disposed.
• Exception aggregation slightly improved: single exception is rethrown as-is.