Fix #1978 — Add XML docs for EventAggregator APIs

[sachPundir]

Description of Change

Added missing XML documentation comments to event-related APIs to improve IntelliSense support and API discoverability.

This PR enhances developer guidance when using Prism's EventAggregator.

Bugs Fixed

• Fixes Add Missing XML Docs #1978

API Changes

None

Behavioral Changes

None

Files Updated

• IEventAggregator
• ThreadOption

PR Checklist

• Has tests (not required for documentation-only changes)
• Rebased on top of master at time of PR
• Changes adhere to coding standard

[dansiegel]

Requesting change: docs are inaccurate for non-UI contexts. The code throws () if SynchronizationContext is null. Please replace the note in (UIThread remarks) so it says this option requires the event aggregator to be constructed on a UI thread; it is not a fallback to publisher thread.

[dansiegel]

Requesting change: ThreadOption.UIThread docs are inaccurate for non-UI contexts. The code throws InvalidOperationException (Resources.EventAggregatorNotConstructedOnUIThread) if SynchronizationContext is null. Please replace the note in src/Prism.Events/ThreadOption.cs (UIThread remarks) so it says this option requires the event aggregator to be constructed on a UI thread; it is not a fallback to publisher thread.