Add list_events on AgentSession and AsyncAgentSession

[bhaveshpatel640]

Note

Low Risk
Additive SDK surface with pass-through to existing API; no auth or core logic changes.

Overview
Adds list_events to AgentSession and AsyncAgentSession, exposing paginated session-wide events (newest first) on the enriched session wrapper instead of only via the raw sessions client.

Both sync and async methods forward to agents.sessions.list_events with page_token, optional last_turn_id (initial window anchor; defaults to the session’s last turn), limit (default 100), and RequestOptions. Return types are SyncPager/AsyncPager of SessionEventItem. Class docstrings now mention list_events alongside the other convenience methods.

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