test(node): handle SSE responses split across chunks

[sjr666666]

Summary

• Read Streamable HTTP SSE responses until the expected events are received instead of assuming one reader.read() contains the full response.
• Add a bounded cancellation timeout so a stalled stream cannot leave the test hanging.
• Keep the production transport unchanged; this is a test reliability fix.

Fixes #2661

Verification

• Node 26.7.0: npx --yes node@26.7.0 node_modules/vitest/vitest.mjs run test/streamableHttp.test.ts — 74 passed.
• Node 24.12.0: pnpm --filter @modelcontextprotocol/node test — 102 passed.
• Node 26.7.0: full @modelcontextprotocol/node workspace tests — 102 passed.
• pnpm --filter @modelcontextprotocol/node typecheck — passed.
• pnpm --filter @modelcontextprotocol/node build — passed.

The repository pre-push hook also runs sync:snippets --check, which currently reports 49 existing out-of-sync files outside this change; no snippet files are touched by this PR.

[changeset-bot]

🦋 Changeset detected

Latest commit: 66d2f69

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/node	Patch

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commit: 66d2f69

[sjr666666]

@felixweinberger — updated the branch onto main so the required checks could re-run; all 14 green, including build, test (20) and test (24). Bumping in case it slipped through: test-only change that makes the node middleware SSE tests accumulate a streamed response across fetch chunks instead of assuming the first read holds a whole event, so chunked-delivery regressions fail the suite rather than passing intermittently. No runtime change, changeset included.