docs: show typed response stream failures

[SiluPanda]

Summary

• update the Responses streaming README snippet and runnable example to switch on stream.Current().AsAny()
• show how to handle ResponseErrorEvent and ResponseFailedEvent
• distinguish typed stream events from transport/API-level failures returned by stream.Err()

Testing

• go -C examples vet ./responses-streaming
• go -C examples test ./responses-streaming
• go -C examples test ./...
• git diff --check

Fixes #585

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-30T17:55:24.529550Z	5f62608	PR opened
🔒 Security Review	✅ Completed 2026-08-30T17:56:24.939513Z	5f62608	PR opened

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[sylvesterkaczmarek]

Checked this against the current streaming shape. AsAny() is the right boundary for the typed error/failed events, while keeping stream.Err() separate avoids implying transport or decode failures arrive as response events.

The completed case also pulls the final text from the terminal response instead of trying to rebuild it from deltas. Looks consistent.