Fix: Skip non-message SSE events in processStream (#637)

Fixes #636

## Problem
The StreamableClientTransport tries to JSON-decode ALL SSE events,
including ping events sent by servers for keep-alive purposes. This
causes errors like 'invalid character 'p' looking for beginning of
value' when encountering non-JSON event data.

## Solution
Modified processStream to check evt.Name and skip events that are not
'message' events (or unnamed events, which default to 'message' per SSE
spec).

## Changes
- Added event name check in processStream before attempting JSON decode
- Added test case for ping event filtering
- Added test for scanning multiple events including ping events

## Testing
- All existing tests pass
- New tests verify ping events are properly filtered
- Tested with DeepWiki MCP server which sends ping events

mcp/streamable.go

@@ -1656,6 +1656,12 @@ func (c *streamableClientConn) processStream(requestSummary string, resp *http.R
 			lastEventID = evt.ID
 		}
 
+		// Skip non-message events (e.g., "ping" events used for keep-alive)
+		// According to SSE spec, events with no name default to "message"
+		if evt.Name != "" && evt.Name != "message" {
+			continue
+		}
+
 		msg, err := jsonrpc.DecodeMessage(evt.Data)
 		if err != nil {
 			c.fail(fmt.Errorf("%s: failed to decode event: %v", requestSummary, err))