fix(ssestream): skip dispatching empty events for comment-only SSE blocks

[jh317729530]

Summary

The eventStreamDecoder.Next() method currently dispatches an event unconditionally whenever it encounters an empty line. This causes issues when SSE keep-alive comments (e.g. : ping) are followed by a blank line — an empty Event (with no type and no data) is dispatched, which leads to a json.Unmarshal error (unexpected end of JSON input) in the upstream Stream[T].Next().

Per the SSE specification:

If the data buffer is an empty string [...] set the data buffer and the event type buffer to the empty string and return.

In other words, if no data: or event: fields were set before the blank line, the event should be silently discarded.

Changes

• packages/ssestream/ssestream.go: Added a guard in eventStreamDecoder.Next() to continue (instead of dispatching) when an empty line is encountered but both data and event are empty.
• packages/ssestream/ssestream_test.go: Added comprehensive tests covering:
  • Comment-only blocks (: ping\n\n) produce no events
  • Comments followed by real data events are handled correctly
  • Multiple consecutive comment blocks are all skipped
  • Events with only a type (no data) still dispatch correctly
  • Normal data events and events with both type and data work as before

Reproduction

Send an SSE stream containing:

: ping

data: {"id":"1"}

Before fix: The : ping\n\n block dispatches an empty event → json.Unmarshal([]byte{}, ...) → error.

After fix: The : ping\n\n block is silently skipped, only the data: {"id":"1"} event is dispatched.

Test plan

• All new unit tests pass (go test ./packages/ssestream/ -v)
• No existing tests are broken
• Maintainers verify behavior against real OpenAI streaming endpoints with keep-alive pings

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