Python: Fix ResponsesAgent image duplication in chat history preparation

[moonbox3]

Motivation and Context

The ResponsesAgent is duplicating messages in chat history preparation when messages contained multiple content items (text + images). The response_inputs.append() call is inside the content processing loop, causing each message to be added once per content item.

The fix:

• Moved response_inputs.append() outside the content processing loop
• Added unit test to verify multiple images don't get duplicated

Description

• Fixed loop placement in _prepare_chat_history_for_request
• Added test test_prepare_chat_history_multiple_images_no_duplication
• Fixes Python: Bug: Responses Agent image count issue #12848

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 😄

[moonbox3]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
agents/open_ai
responses_agent_thread_actions.py	361	100	72%	169, 186, 197, 208–209, 376–378, 392, 397, 404, 422–423, 427, 471–473, 476, 494–495, 504–506, 510, 514–515, 519–521, 532–534, 556, 558–559, 566–567, 569–570, 572, 574–575, 577–578, 676, 681, 689–690, 692, 698, 703, 708–709, 716–718, 723–724, 726–731, 733, 737–738, 740, 744, 748, 753–754, 759, 762–763, 834–837, 878–879, 883–885, 887, 912–913, 915–919, 921, 931–932, 940, 944, 951, 957, 1021
TOTAL	26786	4583	82%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3680	22 💤	0 ❌	0 🔥	1m 53s ⏱️