fix: use active agent name for streaming message sender after handoff

[Osamaali313]

Problem

In Swarm.run_and_stream (swarm/core.py), the per-turn assistant message dict is initialized with the wrong sender:

active_agent = agent
...
while len(history) - init_len < max_turns:
    message = {
        "content": "",
        "sender": agent.name,       # <-- starting agent, not the active one
        "role": "assistant",
        ...
    }

agent is the original starting agent, bound once at entry. During the loop, handoffs reassign active_agent (active_agent = partial_response.agent) but never agent. So every assistant message stored in streaming mode after a handoff is stamped with the starting agent's name.

This is a wrong-variable bug — the same method already uses the correct variable everywhere else:

• The streamed delta: delta["sender"] = active_agent.name
• The non-streaming twin run: message.sender = active_agent.name

Only the streaming message dict uses agent.name.

Impact

Consumers reading Response.messages[*]["sender"] in streaming mode get the wrong agent for any assistant turn after a handoff — including the shipped repl.pretty_print_messages, which prints message["sender"]. Within a single response the streamed delta says one agent while the stored message says another.

Reproduction

Driving run(..., stream=True) through a handoff (agent1 → agent2):

• Streamed delta senders: ['Agent 1', 'Agent 2']
• Before: stored assistant messages: ['Agent 1', 'Agent 1'] — the agent2 turn is mislabeled
• After: stored assistant messages: ['Agent 1', 'Agent 2'] — matches the deltas

Fix

"sender": active_agent.name,

Test

Added test_streaming_message_sender_after_handoff to tests/test_core.py (there was no streaming test). It fails before the fix (stored sender 'Agent 1') and passes after; the full suite is green (7 passed).

Note: I see Swarm is in maintenance mode / superseded by the OpenAI Agents SDK, so feel free to close if out of scope — sharing it as a small, self-contained correctness fix in case it's useful.

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