[docs] ChatModelAgent quickstart uses outdated event field path

**Describe the bug**

The ChatModelAgent quickstart in `README.md` and `README.zh_CN.md` uses:

```go
fmt.Println(event.Message.Content)
```
This does not match the current ADK event structure. In the current API, the message is exposed through event.Output.MessageOutput, so the example is outdated and does not reflect the current code.

**To Reproduce**

Steps to reproduce the behavior:
1. Open the ChatModelAgent quickstart in README.md
2. Follow the example with the current eino / adk API
3. Inspect the event type returned by runner.Query(...)
4. See that event.Message does not exist
**Expected behavior**

The quickstart should use the current API, for example by reading the message from event.Output.MessageOutput and then accessing Content from the returned message.

**Screenshots**

**Version:**

main branch, introduced in README rewrite commit e328752

**Additional context**

This issue exists in both:

- README.md
- README.zh_CN.md

The current ADK types use AgentEvent -> AgentOutput -> MessageOutput.

Find the ChatModelAgent quickstart examples in README.md and README.zh_CN.md. Compare the event returned by runner.Query(...) with the current AgentEvent -> AgentOutput -> MessageOutput API, then update both examples so they access the message through event.Output.MessageOutput and verify the quickstarts match the current ADK types.

Written by the indexing model from the issue text.