feat(streaming): emit ReasoningDeltaEvent for reasoning/thinking deltas

[adityasingh2400]

Summary

When models like o3 or DeepSeek-R1 produce reasoning/thinking tokens during streaming, those deltas currently only surface as raw RawResponsesStreamEvent wrappers around low-level response.reasoning_summary_text.delta or response.reasoning_text.delta events. To consume them, callers have to inspect .data.type and cast the event themselves — there's no clean signal in the StreamEvent union.

This PR adds ReasoningDeltaEvent to StreamEvent and emits it alongside the existing raw event so reasoning deltas are as easy to consume as message deltas.

Closes #825

What changed

• Added ReasoningDeltaEvent dataclass to stream_events.py with delta, snapshot, and type fields
• Updated StreamEvent type alias to include ReasoningDeltaEvent
• Exported from agents/__init__.py
• In run_internal/run_loop.py, the run_single_turn_streamed loop now emits a ReasoningDeltaEvent after each ResponseReasoningSummaryTextDeltaEvent (o-series) and ResponseReasoningTextDeltaEvent (DeepSeek/LiteLLM)
• The snapshot field accumulates the full reasoning text so far in the turn, so callers don't have to maintain their own buffer
• Raw events are still emitted unchanged — fully backwards compatible

Usage example

from agents import Agent, Runner
from agents.stream_events import ReasoningDeltaEvent

agent = Agent(name="thinker", model="o3-mini")
result = Runner.run_streamed(agent, "prove P != NP")

async for event in result.stream_events():
    if isinstance(event, ReasoningDeltaEvent):
        print(event.delta, end="", flush=True)

print()  # reasoning complete

Tests

Added tests/test_reasoning_delta_stream_event.py covering:

• ReasoningDeltaEvent is emitted for reasoning items
• Snapshot grows monotonically and ends with full text
• No event emitted for plain text responses
• Raw events still emitted alongside
• Importable directly from agents
• Correct dataclass fields

Also updated tests/test_stream_events.py::test_complete_streaming_events to account for the new event in the event sequence (count goes from 27 → 28).

Summary by CodeRabbit

• New Features

  • Added ReasoningDeltaEvent for streaming incremental reasoning updates from AI agents, featuring:
    • delta: incremental reasoning fragment
    • snapshot: accumulated reasoning text
    • Emitted during agent streaming when reasoning is available

• Tests

  • Added comprehensive tests validating reasoning delta streaming behavior and event accumulation