Fixes a bug in FetchClient.handleEventStream where SSE events split across TCP chunks would fail to parse.

Problem

TCP chunks don't align with SSE event boundaries. When streaming responses arrive, a single reader.read() call might return partial JSON:

• Chunk 1: data: {"response":"Hel
• Chunk 2: lo"}\n\ndata: {"response":" world"}\n\n

The previous implementation tried to parse each chunk independently, causing JSON.parse to fail on incomplete data.

Solution

Buffer incoming data and only parse complete SSE events (those terminated by \n\n):

let buffer = ''

while (true) {
  const { value, done } = await reader.read()
  if (done) break

  buffer += decoder.decode(value, { stream: true })

  const events = buffer.split('\n\n')
  buffer = events.pop() || '' // Keep incomplete event in buffer

  for (const event of events) {
    // Parse complete events
  }
}

Changes

• fix: Buffer SSE stream chunks before parsing in FetchClient.handleEventStream
• test: Add tests for chunked SSE handling (split events, multiple events per chunk, delimiter boundary splits, multi-byte UTF-8)
• docs: Add "Streaming Responses (SSE)" section to REST client documentation

Testing

All existing tests pass plus 4 new tests:

• handles SSE events split across chunks
• handles multiple events in a single chunk
• handles event split at delimiter boundary
• `handles multi-byte UTF-8 characters split across chunks