aither-adk 2.1.0 — AitherAgent.stream_react

Add AitherAgent.stream_react(message, on_event, history) — the canonical streaming tool loop. Streams <think> reasoning AND the final answer live while executing the agent's registered tools (events: thinking/token/tool/tool_result/done), with a one-shot retry on transient connection drops. chat() (native function-calling) is unchanged. Surfaces wanting live reasoning + tools (web chat, terminals) should use stream_react.