It would be helpful to have an optional, built-in feedback control (thumbs-up / thumbs-down)

'adk web' provides a great development UI and evaluation traces for agent responses. It would be helpful to have an optional, built-in feedback control (thumbs-up / thumbs-down) shown after agent replies so end users and testers can submit quick satisfaction signals.

### Proposal
- Add an optional toggle in the adk-web configuration to show a feedback control (👍 / 👎) after each agent message.
- When feedback is submitted, record an event in the session trace (so evaluators can filter messages by feedback) and emit an HTTP call to a configurable endpoint so deployments can persist feedback.
- For 👎, show an optional short text box for a reason.

### Benefits
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- Faster human feedback loop for improving agents
- Easier to flag low-quality replies in evaluation workflows

### Implementation notes
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- UI should be opt-in via config (so devs can disable for certain runs).
- Add server endpoint `/adk/feedback` (or plumb through existing session event API) that writes a `feedback` event into session trace.
- Minimal design: two icons + thank-you confirmation.