bug: StreamableHTTP SSE parser fails when SSE event lacks "event:" field

[dracarys13]

Description

The StreamableHTTP transport's SSE (Server-Sent Events) parser incorrectly requires both event: and data: fields to process an SSE event. According to the W3C SSE specification, the event: field is optional, and SSE events with only data: fields should be processed normally.

What happened:

• Server responds with a valid SSE stream containing only data: field but no event: field
• The SSE parser recognizes the data: field but doesn't trigger the event handler because no event: field was present
• This results in unexpected nil response error and transport failure

What was expected:

• SSE events with only data: fields should be processed normally
• The event: field should be treated as optional

Code Sample

// Current problematic logic in readSSE function
if line == "" {
    // Empty line means end of event
    if event != "" && data != "" {  // ❌ This incorrectly requires both event AND data
        handler(event, data)
        event = ""
        data = ""
    }
    continue
}

Logs or Error Messages

========= SSE STREAM READING =========
Starting to read SSE stream...
SSE line received: data: {"jsonrpc":"2.0","id":1,"result":{...}}
  -> Parsed data: '{"jsonrpc":"2.0","id":1,"result":{...}}'
Empty line received (event boundary)
SSE stream reached EOF
========= SSE RESPONSE COMPLETED =========
Received nil response from SSE stream           # ❌ Event handler never called
transport error: unexpected nil response

Environment

• Go version: 1.23
• mcp-go version: latest
• OS: macOS
• Architecture: arm64

[rwjblue-glean]

Fixed by #376