fix: handle null body status in createCustomFetch proxy

[evan-choi]

Summary

• Fix TypeError: Response constructor: Invalid response status code 204 when disconnecting from MCP servers that return HTTP 204 No Content on DELETE session termination
• Affects all MCP servers using the Streamable HTTP transport (reproducible with the official Go MCP SDK)

Problem

createCustomFetch wraps node-fetch responses for the SDK, but fails on null body statuses (204, 205, 304):

1. SSE branch crash: headerHolder retains Accept: text/event-stream from prior GET/POST requests. When DELETE fires, the stale header makes isSSE = true, and new Response(webStream, {status: 204}) throws because the Web Response constructor rejects a body for null body statuses.
2. Pass-through branch crash: Even if the SSE branch is skipped, the raw node-fetch response is returned via as unknown as Response. The SDK then calls response.body?.cancel() which doesn't exist on Node.js Readable streams (node-fetch), causing TypeError: response.body?.cancel is not a function.

Fix

Detect null body statuses before the SSE stream conversion check and return a proper Web Response with null body:

const isNullBodyStatus =
  response.status === 204 ||
  response.status === 205 ||
  response.status === 304;

if (isNullBodyStatus) {
  return new Response(null, { status, statusText, headers });
}

Note on 101 Switching Protocols: The Fetch spec also lists 101 as a null body status. It is intentionally excluded here because the body stream carries the upgraded protocol data (e.g., WebSocket), which must not be discarded.

Known limitations / Follow-up

• The stale Accept header leaking from headerHolder into unrelated requests (e.g., DELETE) is a separate root cause that could be addressed in a follow-up PR by scoping header merging per request type.
• Path Completion requests #3 (return response as unknown as Response) is a pre-existing issue where non-SSE, non-null-body responses are not properly converted from node-fetch to Web Response. This could cause silent failures for response.body?.cancel() on other status codes.

Test plan

• npm run prettier-check passes
• npm test passes (487 tests)
• Manual test: connect to MCP server (Go SDK) via Streamable HTTP → disconnect → no errors in proxy log
• Manual test: connect → call tool → disconnect → clean shutdown confirmed