autogen-ext: Support Image generation message on ai agent

[linznin]

Why are these changes needed?

This PR adds support for handling image generation results returned via the OpenAI Responses API in the OpenAI agent message stream.

Previously, the agent primarily relied on response_obj.output_text (or a fallback placeholder) and would therefore miss non-text outputs contained in response_obj.output, such as image generation tool calls. As a result, image-generation-capable models/tools could successfully generate images, but the agent would not surface them to downstream consumers (and message history would not reflect them).

With this change, the agent:

• Parses response_obj.output when present.
• Converts ImageGenerationCall results (base64) into AddImage items so downstream UIs/clients can render images.
• Collects text from ResponseOutputMessage parts and returns TextMessage when output is purely textual.
• Returns a MultiModalMessage when the output includes non-string content (e.g., images), while preserving message history semantics.

This enables image generation responses to be delivered through the same streaming interface and message abstractions used by other modalities.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

❌ Patch coverage is 47.05882% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.18%. Comparing base (13e144e) to head (0bffcf4).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...ext/src/autogen_ext/agents/openai/_openai_agent.py	47.05%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7156      +/-   ##
==========================================
- Coverage   81.22%   81.18%   -0.04%     
==========================================
  Files         244      244              
  Lines       18512    18525      +13     
==========================================
+ Hits        15036    15040       +4     
- Misses       3476     3485       +9     

Flag	Coverage Δ
unittests	81.18% <47.05%> (-0.04%)	⬇️

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[linznin]

@microsoft-github-policy-service agree