docs: clarify dynamic prompts and runtime doc caveats

[seratch]

This pull request updates the SDK documentation for runtime behaviors that were already supported but not clearly explained in the English docs. It adds dynamic prompt callback guidance, clarifies when to use Agent.create(...) for mixed handoff output types, documents callModelInputFilter and sessionInputCallback behavior around persisted history and server-managed conversation deltas, and fills in the Realtime approval flow for session.approve(...) and session.reject(...).

[changeset-bot]

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Latest commit: f8f0cad

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