refactor: add mcp client session manager

[calmini]

Overview

The MCP Client Session Manager is a dedicated component that was separated from the MCPRouter to handle all client session-related operations, so that router will delegate mcp server updating to session manager.

Key Responsibilities

1, maintain client session for mcp servers that could be shared over multiple profiles
2. provide add/remove/update methods to manipulate mcp servers updating
3. provide api to retrieve tool/prompt/resource through server_id parameter and hide the complicated mapping structure

a basic operation for router to call mcp request works as below

conn = client_session_manager.get_session(server_id)
if conn and conn.session:
    result = await conn.session.call_tool(tool.name, req.params.arguments or {})