Instrument `Agent.iter()` with `invoke_agent` span

`Agent.iter()` is a public API for step-by-step agent iteration that does NOT call `Agent.run()`, so currently no `invoke_agent` span is created. Inner model calls still get `gen_ai.chat` spans via the `ModelRequestNode` patch, but without the parent agent span.

This should be patched similarly to `Agent.run_stream()` — wrapping the async context manager to create an `invoke_agent` span and push/pop the agent context.