Add AgentDraftSession as private class

[chiragjn]

Note

Medium Risk
Session inheritance and typing changes touch core turn/session APIs; behavior should stay equivalent for existing AgentSession users, but draft sessions share session endpoints and optional agent_name differs from production sessions.

Overview
Introduces private AgentDraftSession / AsyncAgentDraftSession for draft agent sessions with an inline AgentSpec, plus update(agent_spec) that PATCHes the draft session and refreshes local agent_spec and updated_at.

To support that, AgentSession is refactored into BaseAgentSession / AsyncBaseAgentSession (shared identity + turn/event helpers) and thin AgentSession / AsyncAgentSession subclasses that only add required agent_name. Turn, PreparedTurn, and turn pager wrappers now type against the base session classes so draft sessions can run the same turn flow.

BaseAgentSession and AsyncBaseAgentSession are exported from the public agents package; draft session types stay under agents.private via existing private re-exports.

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