Priming streamable http events use hardcoded event id of: "0"

[glicht]

Describe the bug
From what I am seeing the event ID of priming events of an SSE event stream are always using an event id of: "0". I would expect the first priming event of the stream to contain the "http_request_id" as part of the event id: 0/<http_request_id> so it will be possible to resume the stream if disconnected.

To Reproduce
Run the counter example server:

cargo run -p mcp-server-examples --example servers_counter_streamhttp

Then run the following bash script with curl:

#!/usr/bin/env bash
set -euo pipefail

URL="${1:-http://localhost:8000/mcp}"

# POST initialize and capture session ID from response header
SESSION_ID=$(curl -s -D - -o /dev/null \
  -X POST "$URL" \
  -H "Accept: application/json, text/event-stream" \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
      "protocolVersion": "2025-06-18",
      "capabilities": {},
      "clientInfo": { "name": "test-client", "version": "1.0.0" }
    }
  }' | grep -i "^mcp-session-id:" | awk '{print $2}' | tr -d '\r')

if [[ -z "$SESSION_ID" ]]; then
  echo "ERROR: No Mcp-Session-Id header in response" >&2
  exit 1
fi

echo "Session ID: $SESSION_ID"
export MCP_SESSION_ID="$SESSION_ID"

# POST notifications/initialized to complete the MCP handshake
STATUS=$(curl -s -o /dev/null -w "%{http_code}" \
  -X POST "$URL" \
  -H "Accept: application/json, text/event-stream" \
  -H "Content-Type: application/json" \
  -H "Mcp-Session-Id: $MCP_SESSION_ID" \
  -H "Mcp-Protocol-Version: 2025-06-18" \
  -d '{
    "jsonrpc": "2.0",
    "method": "notifications/initialized"
  }')

if [[ "$STATUS" != "202" ]]; then
  echo "ERROR: notifications/initialized returned $STATUS, expected 202" >&2
  exit 1
fi

echo "Handshake complete (202 Accepted)"
echo "MCP_SESSION_ID=$MCP_SESSION_ID"

for i in {1..2}; do
  echo "Calling tool ... (iteration $i)"
  
  curl 'http://127.0.0.1:8000/mcp' \
    -H 'accept: application/json, text/event-stream' \
    -H 'content-type: application/json' \
    -H "Mcp-Protocol-Version: 2025-06-18" \
    -H "mcp-session-id: $MCP_SESSION_ID" \
    --data-raw '{"method":"tools/call","params":{"name":"say_hello","arguments":{},"_meta":{"progressToken":1}},"jsonrpc":"2.0","id":'"$i"'}'

done

Output shows that both tool calls contain a priming event with "id: 0":

Session ID: ef43085b-0b2b-4b9f-ba59-6e5d26118376
Handshake complete (202 Accepted)
MCP_SESSION_ID=ef43085b-0b2b-4b9f-ba59-6e5d26118376
Calling tool ... (iteration 1)
data: 
id: 0
retry: 3000

data: {"jsonrpc":"2.0","id":1,"result":{"content":[{"type":"text","text":"hello"}],"isError":false}}
id: 0/0

Calling tool ... (iteration 2)
data: 
id: 0
retry: 3000

data: {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"hello"}],"isError":false}}
id: 0/1

Expected behavior
Priming event id of: "0/<http_request_id>"

Additional context
From the spec: https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#resumability-and-redelivery

Event IDs SHOULD encode sufficient information to identify the originating stream, enabling the server to correlate a Last-Event-ID to the correct stream.