feat(agents): add AgentSession for stateful multi-turn async conversations

[ks93]

Summary

• Adds AgentSession class (cognite/client/data_classes/agents/session.py) that stores agent external ID, actions, and conversation cursor for multi-turn async chat.
• Adds AgentsAPI.create_session() as a plain def factory method (not async def) — this keeps it out of the codegen sync path, so the sync client is untouched.
• AgentSession.chat() automatically threads the cursor from each response into the next request; callers manage only messages, not cursor state.

Behavior

• Cursor advances only on a successful response — a failed await session.chat(...) leaves the cursor intact so the call can be retried.
• A null/missing cursor in the response retains the last known cursor.
• Unknown kwargs to create_session() are silently accepted for forward compatibility.
• AgentSession is exported from cognite.client.data_classes.agents.

Test plan

• 16 new async unit tests in tests/tests_unit/test_api/test_agents.py covering cursor threading, failure isolation, null-cursor retention, actions forwarding, action-result round-trips, ToolConfirmationCall pass-through, and scope guards (read-only cursor, sync client exclusion, no reset method)
• Sync client verified: hasattr(sync_client.agents, "create_session") is False
• Codegen verified: _sync_api/agents/agents.py has no new methods (hash-only update)
• All pre-commit hooks pass (ruff, mypy, custom-checks, sync-codegen, pydoclint)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.79%. Comparing base (43efd75) to head (9148112).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2593      +/-   ##
==========================================
+ Coverage   92.76%   92.79%   +0.02%     
==========================================
  Files         480      481       +1     
  Lines       48511    48685     +174     
==========================================
+ Hits        45003    45176     +173     
- Misses       3508     3509       +1     

Files with missing lines	Coverage Δ
cognite/client/_api/agents/agents.py	100.00% <100.00%> (ø)
cognite/client/_sync_api/agents/agents.py	100.00% <ø> (ø)
cognite/client/data_classes/agents/__init__.py	100.00% <100.00%> (ø)
cognite/client/data_classes/agents/session.py	100.00% <100.00%> (ø)
tests/tests_unit/test_api/test_agents.py	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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