AGE-1320: Add internal list sessions to SDK

[chiragjn]

Note

Medium Risk
Refactors how AgentSession performs turn operations (delegation to SessionMixin) while adding new listing/wrapping paths; regressions could affect turn pagination or session references, though tests cover key behaviors.

Overview
Adds caller-scoped agent session APIs on top of existing private gateway endpoints: PrivateAgentSessionClient / AsyncPrivateAgentSessionClient for creating and fetching draft sessions, listing drafts, and list_owned_sessions (saved sessions and drafts in one paginated stream). Raw list items are wrapped as AgentSession or AgentDraftSession using a type discriminant ("session" vs "session/draft").

AgentDraftSession mirrors saved-session ergonomics (including agent_spec) by sharing turn behavior through new SessionMixin / AsyncSessionMixin. AgentSession now delegates prepare_turn, list_turns, get_turn, cancel, and list_events to the mixin instead of inlining client calls and pager wrapping. PreparedTurn, Turn, and related types accept either session wrapper so turn.session stays the enriched owner.

New symbols are exported from agents. Custom tests cover owned-session discriminant wrapping, pagination, draft listing, and that turns keep the enriched session reference.

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