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

[moonbox3]

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.

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 Python: AzureChatCompletion service fails with "Missing required parameter" error #10317

Contribution Checklist

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

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
semantic_kernel/agents/channels
chat_history_channel.py	79	2	97%	167, 170
TOTAL	17094	1861	89%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3048	4 💤	0 ❌	0 🔥	1m 21s ⏱️