fix(server): leave SSE connection policy to adapters

[colinaaa]

Summary

• remove Connection: keep-alive from the three Web Standard Streamable HTTP SSE response paths
• assert that POST, standalone GET, and Last-Event-ID replay responses do not set hop-by-hop connection policy
• add a patch changeset for @modelcontextprotocol/server

Fixes #2553.

Why

A web-standard Response does not own the underlying connection and cannot know its lifetime. Setting Connection: keep-alive in the transport suppresses concrete HTTP/1.1 servers from advertising their real Keep-Alive: timeout=..., and Connection is forbidden on HTTP/2.

Omitting the header does not disable persistent connections. Node HTTP/1.1 can emit its own matching Connection and Keep-Alive headers, while HTTP/2 and other Fetch adapters remain free to apply their own policy. This adds no Connection: close, retry, or timeout behavior.

Verification

• focused Streamable HTTP tests: 47/47
• full @modelcontextprotocol/server tests: 448/448
• server typecheck
• server ESLint and Prettier
• repository pre-push build, typecheck, lint, and snippet checks
• git diff --check

Interaction with #2541

#2541 edits the same three header objects to add SSE heartbeat and anti-buffering behavior. The changes are semantically independent: retain its heartbeat and X-Accel-Buffering changes, while omitting only the hop-by-hop Connection header.

[changeset-bot]

🦋 Changeset detected

Latest commit: 81a5558

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

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/client	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core-internal	Patch

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[pkg-pr-new]

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commit: 81a5558

[knoal]

Reviewing via MCE A/B pilot 7 (sophia@hermes.local).

Summary

Removes Connection: keep-alive header from 3 SSE response sites in streamableHttp.ts. Reason: hop-by-hop connection policy should be the HTTP adapter's job, not the SDK's.

What's good

• Correct technical fix — Connection is a hop-by-hop header per RFC 7230 §6.1, and the SDK was setting it manually when the HTTP adapter should manage connection lifecycle.
• Patch-level changeset is appropriately terse (5 lines).
• Tests updated to no longer expect the header.

APPROVE — clean focused fix.

— sophia