Stop delivering stream events after the caller aborts

[RGB-loop]

Problem

consumeEventStream (used by streamEvents and createSessionStream) checks options.signal between reads, but not between events decoded from the same chunk. When one network chunk carries several events and an onEvent callback aborts after the first, the remaining events decoded from that same chunk are still delivered to onEvent. If the same events arrive in separate chunks, only the first is delivered. So what a caller sees after abort() depends on how the transport happens to split the stream.

This only affects callers that abort from inside onEvent/onSession (or code those callbacks call synchronously). Aborts from outside the callbacks, such as an effect cleanup, already land between reads and were handled correctly before this change.

Reproduction (before this change, main afa07974)

Injected fetch returning two valid agent.session.turn.output_text.delta frames; onEvent calls controller.abort(reason):

Body	Delivered
both frames in one chunk	evt_1, evt_2
one frame per chunk	evt_1

Change

Check the signal before dispatching each decoded event (throwIfAborted, same behavior as the existing post-read check: the call rejects with the abort reason and the reader is cancelled). No protocol or decoder change.

Validation

• New regression test in client.test.ts: fails on main (expected ['evt_1','evt_2'] to deeply equal ['evt_1']), passes with the change; it also asserts the rejection reason and reader cancellation.
• pnpm --filter @oac/agents-client test: 22 files, 745 tests passed.
• pnpm --filter @oac/agents-client typecheck: passed.

Limits: verified with injected fetch/ReadableStream only; apps/web tests were not run because the change only affects behavior after the caller has already aborted.

🤖 Generated with Claude Code

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