AGE-1122: Align public API docstrings across agent SDK code

[bhaveshpatel640]

AGE-1122: Align public API docstrings across manual agent SDK code

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Note

Low Risk
Documentation-only changes with no logic, types, or wire behavior modified.

Overview
Adds NumPy-style docstrings across the manual agent SDK surface so public APIs document parameters, return types, and streaming behavior consistently.

Coverage spans sync and async paths: AgentSession / AsyncAgentSession, AgentSessionClient, PreparedTurn / AsyncPreparedTurn, Turn / AsyncTurn, delta helpers in event_delta, TurnStreamData, and the gateway agents accessors on TrueFoundryGateway / AsyncTrueFoundryGateway. Several one-line docstrings on methods like refresh and stream are expanded to the same Parameters / Returns / Yields pattern, including notes on pagination, request_options, SSE resume via after_sequence_number, and when turn fields are available before vs after execute().

No runtime or API behavior changes—documentation only.

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