ChatDatabricks Responses API streaming emits the final text twice

[MarcusKoolaard]

_convert_responses_api_chunk_to_lc_chunk suppresses the terminal full-text block only when the immediately previous chunk is response.output_text.delta. Providers close a message with response.output_text.done and response.content_part.done first, so the check never fires and the completed text is appended on top of the streamed deltas.

Repro (no network, released 0.20.0) — feeds the documented event order through _stream with a stubbed client:

EVENTS = [
    ResponseTextDeltaEvent.model_construct(type="response.output_text.delta", item_id="msg_1", delta="hello"),
    ResponseTextDeltaEvent.model_construct(type="response.output_text.delta", item_id="msg_1", delta=" world"),
    ResponseTextDoneEvent.model_construct(type="response.output_text.done", item_id="msg_1", text="hello world"),
    ResponseContentPartDoneEvent.model_construct(type="response.content_part.done", item_id="msg_1",
        part=ResponseOutputText.model_construct(type="output_text", text="hello world", annotations=[])),
    ResponseOutputItemDoneEvent.model_construct(type="response.output_item.done",
        item=ResponseOutputMessage.model_construct(type="message", id="msg_1",
            content=[ResponseOutputText.model_construct(type="output_text", text="hello world")])),
]
llm = ChatDatabricks(model="m", use_responses_api=True)  # client stubbed to yield EVENTS
merged = functools.reduce(operator.add, llm.stream("hi"))

Actual:

[{'type': 'text', 'text': 'hello'}, {'type': 'text', 'text': ' world'}, {'type': 'text', 'text': 'hello world', 'annotations': []}]
joined -> 'hello worldhello world'

Expected: 'hello world'.

Also observed live against Unity AI Gateway (use_ai_gateway_native_api=True, Azure Foundry GPT-5.6), where every agent reply came back doubled.

Fix: treat response.output_text.done and response.content_part.done with a matching item_id as "already streamed" alongside response.output_text.delta. Existing unit tests only cover a delta immediately preceding output_item.done, which is why this passes CI. Happy to send a PR.