ENG-742: Count turns without tools-calls

[ea-rus]

Continue of mindsdb/cowork-server#203

Problem

_turn_count is seeded by counting role == "user" messages in history
(session.py, commands/session.py, memory/history_store.py). Once a host
replays tool activity into history, tool_result messages — which are also
role == "user" — get counted as turns. Every tool call inflates the count,
and the drift compounds as the session is rebuilt each turn.

Impact: the "every 5 turns" identity extraction misfires, telemetry/episodic
turn IDs are wrong, and resume/list show inflated turn totals.

Fix

Add is_user_turn(message) in history_store.py: a user-role message whose
content is only tool_result blocks is a tool reply, not a turn. Replace all
three naive counts with it.

• One shared helper, no duplicated predicate.
• Backward compatible: histories without tool_result count unchanged.

Fixes https://linear.app/mindsdb/issue/ENG-742/cowork-tool-calls-are-not-stored-in-conversation-history