Add MessageHistory.count() API

[tylerhutcherson]

Summary

Add count() support to message history APIs for efficient message totals without fetching full records.

Level: Beginner

Current State

• Message history retrieval methods exist, but there is no direct count helper.
• Users must fetch messages or build custom query logic to count.

Proposed Change

• Add count methods to:
  • BaseMessageHistory
  • MessageHistory
  • SemanticMessageHistory
• Use efficient count query paths and support session scoping.

Definition of Done

• count() returns correct values for current and specified session scopes.
• Integration tests validate counts after add/drop/clear flows.
• No behavioral regressions in existing message APIs.

Out of Scope

• Full analytics/event aggregation features.