test(node): make SSE tests robust to chunked delivery

[DrMkdaddy]

Summary

packages/middleware/node/test/streamableHttp.test.ts assumed every SSE event for a response arrives in a single reader.read() chunk. On newer Node versions fetch can deliver those events in separate chunks, so the tests fail even though the transport behavior is correct.

Fixes #2661.

Changes

• Add readSSEEventUntil(reader, predicate), which accumulates chunks with a streaming TextDecoder until the predicate matches or the stream ends.
• Use it in the two affected tests:
  • should handle batch request messages with SSE stream for responses
  • should store and replay MCP server tool notifications (including the reconnect read)

Testing

• Reproduced on Node v26.8.2: pnpm --filter @modelcontextprotocol/node test -- test/streamableHttp.test.ts → 2 failed, 100 passed.
• After the change: 102 passed.
• pnpm --filter @modelcontextprotocol/node lint passes (eslint + prettier).
• pnpm build:all succeeds.

AI assistance

Drafted with AI assistance; I ran the reproduction, reviewed every line, and verified the fix against the full package suite.

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