runner/runner.go:Run() yielding improvements

**Describe the Bug:**

I'm not sure if you consider this a bug, I _kind of_ do, it's at least a gap

1. It's very helpful for event stream consumers to get the user message back. This allows me to avoid optimistic updates in the UI because the turnaround time is fast enough that it's nearly as good and waaaay simpler
2. Setting some "final" fields based on the function call

**Steps to Reproduce:**

I made the following changes to `runner/runner.go` to enable this

https://github.com/google/adk-go/compare/main...verdverm:adk-go:veg#diff-20e6696d056b9497806d8852a99dad01af18dab2c3378ebf73dee3ccbea93f45