fix(streaming): accumulate response.reasoning_text.delta in ResponseStreamState

[yen0304]

Summary

When streaming a response that includes a reasoning output item (type "reasoning"), the Python SDK was silently discarding two events:

1. response.content_part.added — when a reasoning_text part is added to a reasoning output item, the SDK only handled output.type == "message" and fell through without appending anything for output.type == "reasoning".
2. response.reasoning_text.delta — this event type was entirely unhandled in accumulate_event, so text deltas were never accumulated.

The result: after streaming a response with reasoning, snapshot.output[i].content remains None and no text is ever built up — even though the wire events arrived correctly.

The TypeScript SDK handles both cases correctly (see _events.ts):

• response.content_part.added appends a ReasoningItemContent when output.type === "reasoning"
• response.reasoning_text.delta accumulates content.text += event.delta

This PR mirrors that logic in the Python SDK.

Changes

• src/openai/lib/streaming/responses/_responses.py: extend accumulate_event to handle both cases
• tests/lib/responses/test_responses.py: add test_stream_state_accumulates_reasoning_text_delta which exercises the full sequence (response.created → output_item.added → content_part.added → three reasoning_text.delta events) and asserts the accumulated text

Test

def test_stream_state_accumulates_reasoning_text_delta() -> None:
    # drive ResponseStreamState through a reasoning stream sequence
    # and assert that content[0].text == "Let me think carefully."