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

[ericapisani]

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.

[linear-code]

PY-2241