fix: skip empty-data SSE events per HTML5 spec

[Chronostasys]

Problem

Some LLM providers (e.g. Xiaomi MiMo) send keep-alive comments during streaming:

: PROCESSING

This produces an SSE event with empty Data. The current eventStreamDecoder.Next() dispatches it, and Stream[T].Next() then calls json.Unmarshal([]byte{}, ...) which returns "unexpected end of JSON input" — crashing the stream.

Root Cause

The HTML5 SSE specification states:

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

Events with empty data should be silently discarded. The current implementation dispatches them regardless.

Fix

In eventStreamDecoder.Next(), when a blank line triggers event dispatch, check if data.Len() == 0. If so, reset state and continue instead of returning the empty event.

Testing

Added ssestream_test.go with 5 test cases:

• Comment-only event is skipped
• Multiple comments before data are skipped
• Multiple data events are preserved
• Stream-level integration: comment + data works end-to-end
• Comment-only stream produces no events and no error

All tests pass.

Compatibility

• Normal OpenAI/Anthropic streams: no change (they never send empty-data events)
• Providers using SSE comments as keep-alive: now work correctly
• No API changes, fully backward compatible