[sharosoo]

Fixes #237.

Changes

• _toolset.py: Preserve ToolReturn (return_value, content, metadata) via a typed tool_return: ToolReturn | None = None field on RestateContextRunResult, and preserve CallDeferred.metadata / ApprovalRequired.metadata via metadata: dict[str, Any] | None = None. Default both fields to None so existing journal entries continue to deserialize.
• _toolset.py & turnstile.py: Call turnstile.allow_next_after(id) and do not cancel sibling tool calls when a tool raises CallDeferred, ApprovalRequired, or ModelRetry. Ignore untracked tool_call_id values in Turnstile.wait_for and cancel_all_after (such as resumed deferred tool calls before the first model request or nested tool calls) unless the turnstile is already canceled.
• _model.py: Initialize current_state().turnstile in RestateModelWrapper.request_stream just as in request (also addresses pydantic ext: request_stream() never initializes the turnstile — KeyError('call_...') on first tool call of a streamed run #223).
• _agent.py: Override RestateAgent.iter() with with self._restate_overrides(): and matching output-type overloads so agent.iter() uses the durable model and toolset wrappers.
• tests/test_pydantic_ext.py: Add 13 unit tests covering ToolReturn round-trip, CallDeferred metadata, parallel ApprovalRequired / CallDeferred / ModelRetry sibling execution, request_stream turnstile initialization, resumed deferred tool calls, untracked turnstile IDs, and durable agent.iter().

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