test(node): read SSE streams until expected events arrive - #3099

Several streamableHttp tests called reader.read() once and asserted that multiple SSE events
shared one fetch chunk. On Node 26 fetch can deliver them in separate chunks, so the tests
failed although the transport was correct. Add a readSSEUntil helper and use it in the two
affected tests.

Fixes #2661.

Motivation and Context

packages/middleware/node/test/streamableHttp.test.ts assumed that every SSE event written by
the server arrives in the first reader.read() result. That is a property of how the runtime
chunks the response body, not of the transport. On Node v26.7.0 the events can be split across
chunks, and two tests fail before reading the later events:

• should handle batch request messages with SSE stream for responses reads only the req-1
  event and fails before req-2.
• should store and replay MCP server tool notifications reads an earlier {} event and fails
  before "Second notification from MCP server".

The change:

• Adds a readSSEUntil(reader, expected, timeoutMs) test helper. It reads until the
  accumulated text contains every expected string. It throws, including the text received so far,
  if the stream ends early or the 5s timeout is hit.
• Uses it in both tests above, including the post-reconnect read in the replay test.
• In the replay test, takes the Last-Event-ID from the event that carries the "First
  notification" message. Before, it took the first id: in the chunk, which could belong to the
  earlier {} event.

Test-only change. No library code is touched, so no changeset.

How Has This Been Tested?

• pnpm build:all
• pnpm --filter @modelcontextprotocol/node test -- test/streamableHttp.test.ts passes (all 5
  test files in the package, 105 tests) on Node v22.22.2.
• prettier --check is clean on the changed file.
• I have not run this on Node 26, where the issue was reported, so I could not reproduce the
  original failure. The helper removes the single-chunk assumption, but confirmation on Node 26
  (locally or in CI) would be welcome.

Breaking Changes

None. Test-only change.

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

Only the two tests named in the issue are changed. Other tests in this file also call
reader.read() once, but they don't assert on several events at a time, so I left them alone
to keep the change small.

• "I have read the MCP Documentation": I left it unticked. Tick it only if you've read it.
• "Tests pass locally": I ticked it, but it only covers the node package's tests. After your
  push, the pre-push hook will have run lint, build and typecheck, so tick it only if the hook
  passed.
• Documentation box: left unticked on purpose, since none is needed.