test_streaming_elicitation: use proper SSE instead of NDJSON

[howardjohn]

Motivation and Context

NDJSON is not acceptable in the spec.

Before:

$ curl -iN --max-time 5 -H 'Content-Type: application/json' -H 'Accept: application/json, text/event-stream' -H 'MCP-Protocol-Version: 2026-07-28' -H 'Mcp-Method: tools/call' -H 'Mcp-Name: test_streaming_elicitation' --data '{
      "jsonrpc":"2.0",
      "id":1,
      "method":"tools/call",
      "params":{
        "name":"test_streaming_elicitation",
        "arguments":{},
        "_meta":{
          "io.modelcontextprotocol/protocolVersion":"2026-07-28",
          "io.modelcontextprotocol/clientInfo":{"name":"conformance-client","version":"1.0.0"},
          "io.modelcontextprotocol/clientCapabilities":{}
        }
      }
    }' http://localhost:3001/mcp
HTTP/1.1 200 OK
X-Powered-By: Express
Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: Mcp-Session-Id
Content-Type: application/json
Transfer-Encoding: chunked
Date: Mon, 06 Jul 2026 16:30:12 GMT
Connection: keep-alive
Keep-Alive: timeout=5

{"jsonrpc":"2.0","method":"notifications/progress","params":{"progressToken":"token-abc","total":100,"value":50}}
{"jsonrpc":"2.0","id":1,"result":{"content":[{"type":"text","text":"Streaming complete"}]}}

After:

$ curl -iN --max-time 5 -H 'Content-Type: application/json' -H 'Accept: application/json, text/event-stream' -H 'MCP-Protocol-Version: 2026-07-28' -H 'Mcp-Method: tools/call' -H 'Mcp-Name: test_streaming_elicitation' --data '{
      "jsonrpc":"2.0",
      "id":1,
      "method":"tools/call",
      "params":{
        "name":"test_streaming_elicitation",
        "arguments":{},
        "_meta":{
          "io.modelcontextprotocol/protocolVersion":"2026-07-28",
          "io.modelcontextprotocol/clientInfo":{"name":"conformance-client","version":"1.0.0"},
          "io.modelcontextprotocol/clientCapabilities":{}
        }
      }
    }' http://localhost:3001/mcp
HTTP/1.1 200 OK
X-Powered-By: Express
Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: Mcp-Session-Id
Content-Type: text/event-stream
Cache-Control: no-cache
Date: Mon, 06 Jul 2026 16:32:49 GMT
Connection: keep-alive
Keep-Alive: timeout=5
Transfer-Encoding: chunked

event: message
data: {"jsonrpc":"2.0","method":"notifications/progress","params":{"progressToken":"token-abc","total":100,"value":50}}

event: message
data: {"jsonrpc":"2.0","id":1,"result":{"content":[{"type":"text","text":"Streaming complete"}]}}

How Has This Been Tested?

curl above + running the server through a real client that rejected the invalid NDJSON

Breaking Changes

No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context