Add reaction Filter to getReactionCounts

[Silent-Watcher]

Feature: Filter getReactionCounts by Reaction Type

Description:
Extend the getReactionCounts method to accept an optional array of reaction types to filter by. Instead of returning all counts, it only returns counts for the specified reactions.

Acceptance Criteria:

• Method signature: Model.getReactionCounts(reactableId, reactions?: string[]).
• If reactions array is provided, the aggregation pipeline filters to only those types.
• If reactions is undefined, behavior is unchanged (returns all).
• Method is fully typed.
• Tests are added for the new filtered behavior.
• Add usage examples in docs