Feature: add session message count endpoint for efficient counting

[sjawhar]

Summary

Add a dedicated session message count API endpoint and backing session function so clients can retrieve message counts without streaming/deserializing full message history.

Problem

Counting messages currently requires loading all messages, which adds unnecessary overhead and latency when only the count is needed.

Proposed solution

• Add a Session.messageCount() helper using SELECT COUNT(*)
• Expose GET /session/:sessionID/message/count
• Return a lightweight count payload with existing auth/validation/error patterns

Context

This issue is to track and link the existing implementation proposed in PR #13795.

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