Handle keepalive events in Responses streams

[koriyoshi2041]

Problem
Responses streaming can receive keepalive SSE events on long-running requests. They do not carry Response state, but the accumulator currently falls through to the exhaustive error path and ends the stream. Fixes #1964.

Fix
Treat keepalive as a no-op accumulator event, alongside other stream events that do not map onto the Response snapshot. Unknown event types still go through the existing error path.

Test

• pnpm test tests/lib/ResponseAccumulator.test.ts
• pnpm exec prettier --check src/lib/responses/ResponseAccumulator.ts tests/lib/ResponseAccumulator.test.ts
• pnpm build
• git diff --check

Risk
Low. This only ignores the explicit keepalive event; it does not change handling for normal response deltas or undocumented unknown event types.

[HAYDEN-OAI]

thanks for the thoughtful fix and the focused regression test. this is a nice, narrowly scoped way to handle keepalive events while preserving the error path for genuinely unknown events. great work!