Add IObservableMap implementation to windows-collections

[Copilot]

• Fix unused import in generic_guids.rs
• Replace custom handler storage in StockObservableMap with Event<MapChangedEventHandler<K,V>> from windows-core
• Replace custom handler storage in StockObservableVector with Event<VectorChangedEventHandler<T>> from windows-core
• Fix StockMapChangedEventArgs::Key() to return Err(Error::from(E_BOUNDS)) for Reset events (not Error::empty() which has HRESULT 0 = S_OK, causing COM to return Ok(0) and breaking the map_changed_event test assertion)
• Merge fire_changed + fire_changed_reset into a single method taking Option<K::Default>

[kennykerr]

@copilot fix tests

[Copilot]

@copilot fix tests

Fixed in adb3e7c. The map_changed_event test was storing keys as i32 and using args.Key()?, which caused the handler to exit early for Reset events (where Key() returns an error since there's no meaningful key). Changed the event storage to Option<i32> and used args.Key().ok() so the handler always succeeds, recording None as the key for Reset.