fix: ignore sse comment frames

[drewstone]

Summary

• ignore comment-only SSE frames in the OpenAI-compatible backend parser
• prevent control frames like : connected / : keepalive from becoming assistant text
• add regression coverage for comment frames around normal chat deltas

Verification

• pnpm exec vitest run tests/runtime.test.ts tests/backends-openai-usage.test.ts
• pnpm typecheck
• pnpm test
• pnpm lint
• pnpm build