shared sink semantics for *_stream apis.

consumers can opt into three sinks via :into:

• default (no :into) — {:ok, stream}, lazy Stream.
• :into is a pid — events are forwarded as {:openrouter_event, ref, event}; returns {:ok, ref}.
• :into is a fun/2 — runs Enum.reduce/3 for the consumer with acc: opts[:acc] || []; returns {:ok, acc}.

the streaming function is passed as a thunk so we can decide whether to invoke it inline or inside a separate task — this matters because the underlying stream reads from the mailbox of whichever process opened it.