Python: intro allowed content types in chat history channel receive. Add mixed chat image sample. - #10347

…Add mixed chat image sample. (microsoft#10347)

### Motivation and Context

During a group chat, any file reference content created by an assistant
agent, doesn't need to be communicated to a chat completion agent.
Filter these types out and only include other types, like text, if
available.

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### Description

This PR:
- Adds a mixed chat image sample to have an assistant agent generate an
image, along with text, and call the chat completion agent successfully
with only the allowed types (like text).
- Adds a unit test to exercise the same behavior.
- Closes microsoft#10317

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### Contribution Checklist

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- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
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and the [pre-submission formatting
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raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄