Fix Streaming Error in examples/helloworld

[andy010629]

This PR addresses an issue where running the examples/helloworld example results in a streaming-related error. The changes ensure a smoother developer experience when executing the helloworld example by:

1. Enabling Streaming Mode in Agent Card: Updated the Agent Card in the examples/helloworld directory to explicitly enable streaming mode, resolving the error encountered during execution.
2. Adding Comments in Test Client: Added clarifying comments to the test_client.py script to guide developers on the expected behavior and configuration when running the helloworld example.

• [O] Follow the CONTRIBUTING Guide.
• [O] Make your Pull Request title in the https://www.conventionalcommits.org/ specification.
• [O] Ensure the tests and linter pass (Run nox -s format from the repository root to format)
• [O] Appropriate docs were updated (if necessary)

[kthota-g]

hi, this fix is already part of #15