Python: Bug: streaming chunk merge adds an item to every matching item, duplicating text #14786

Describe the bug

When two streaming chunks are added, StreamingContentMixin._merge_items_lists merges each incoming item into every compatible item in the list, not just the first one. The inner loop sets added = True after a successful item + other_item, but it doesn't break, so it keeps going and adds the same incoming item to every later item of the same type that accepts it.

This happens whenever a merged message holds two items of the same type that can both take the next chunk. One case is two StreamingTextContent items with the same choice_index, which the Google AI connector builds when a candidate has more than one text part (google_ai_chat_completion.py#L366-L376). Another is two FunctionCallContent items whose id/index don't tell them apart. Then the next chunk's text or arguments are copied into both.

To Reproduce

semantic-kernel 1.47.0. The file is the same on main (25bd1bb).

from semantic_kernel.contents import AuthorRole, StreamingChatMessageContent, StreamingTextContent

c1 = StreamingChatMessageContent(role=AuthorRole.ASSISTANT, choice_index=0, items=[
    StreamingTextContent(choice_index=0, text="Part one. "),
    StreamingTextContent(choice_index=0, text="Part two. "),
])
c2 = StreamingChatMessageContent(role=AuthorRole.ASSISTANT, choice_index=0, items=[
    StreamingTextContent(choice_index=0, text="tail"),
])
print([i.text for i in (c1 + c2).items])

Output:

['Part one. tail', 'Part two. tail']

Expected behavior

The incoming item is merged into one item only, so the text isn't duplicated: ['Part one. tail', 'Part two. '].

Platform

• Language: Python
• Source: semantic-kernel 1.47.0 (pip) / main at 25bd1bb
• AI model: n/a (no model call needed)
• IDE: n/a
• OS: Windows 11, Python 3.12

Additional context

The fix is a break after added = True. I have it with a regression test in tests/unit/contents/test_streaming_chat_message_content.py and will open a PR.