Python: Fix the async ChatResponse aggregation example

[LI (ktz03)]

Motivation & Context

The ChatResponse.from_update_generator example imports a ChatClient placeholder that the package does not export and uses the legacy get_streaming_response name. Copying it currently fails at the import.

Description & Review Guide

• What are the major changes? Replace the obsolete client placeholder with a self-contained async generator of ordinary text updates using the current public types.
• What is the impact of these changes? Only the example docstring changes; APIs and aggregation behavior remain unchanged. The exact corrected snippet produces Hello How can I help you?, and the adjacent synchronous example still works. Current-config Ruff checks and source compilation pass. No provider/model, package build or full-suite run was performed.
• What do you want reviewers to focus on? The async example uses the existing ChatResponseUpdate, Content.from_text and ChatResponse.from_update_generator APIs correctly.

Related Issue

N/A - trivial change. This is an obvious correction to an example that imports a non-exported client placeholder and uses a legacy streaming method. It uses existing public types and a self-contained async generator without changing any API or runtime behavior.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: Documentation, example verification, and this PR description.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

🟢 Approval recommended

The documentation-only correction uses the existing public APIs correctly and has no unresolved findings.

No open issues 🎉

What changed in this PR

Corrects the async ChatResponse aggregation example without changing APIs or runtime behavior.

Changes:

• Replaces the obsolete client placeholder with a self-contained async generator using public types.
• Shows the expected aggregated text output.

File	Description
python/​packages/​core/​agent_framework/​_types.py	Updates the from_update_generator docstring example.

_{Review effort: Balanced}