docs: Fixed streaming message types

[martimfasantos]

Description

• Fixed incorrect class names in docs/tutorials to match the actual SDK types:
  • Replaced SendStreamingMessage... → SendMessageStreaming...
• Moved the HelloWorldAgent to examples/helloworld/agent.py for better separation of concerns in the example

---

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• Appropriate docs were updated (if necessary)

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[pstephengoogle]

Actually, I misread this. Sorry these docs are supposed to be fixed to the next branch where the next iterations are being developed. This will all by pushed to the main branch in the coming days. We will work to ensure the docs are appropriately aligned, sorry for the confusion

[holtskinner]

Verify that this doesn't conflict with the changes made in #518