fix(server): loosen Accept validation when enableJsonResponse is true

[Zelys-DFKH]

Summary

`WebStandardStreamableHTTPServerTransport` returns 406 when a client sends `Accept: application/json` (without `text/event-stream`) even when the transport is constructed with `enableJsonResponse: true`.

That's the wrong check. With `enableJsonResponse: true`, the server responds with a plain JSON body and never opens an SSE stream. Requiring the client to advertise `text/event-stream` is unnecessary: the media type isn't used, and it breaks clients that send a minimal `Accept: application/json` (a standard JSON-RPC default). The AdCP ecosystem currently works around this with an Express middleware that rewrites the header before the transport sees it.

`handlePostRequest` now reads `this._enableJsonResponse` when validating the Accept header:

• `enableJsonResponse: true` → require only `application/json`
• `enableJsonResponse: false` (default) → require both `application/json` and `text/event-stream`, same as before

This change doesn't touch the GET handler or the SSE response path. `NodeStreamableHTTPServerTransport` is covered as well since it delegates request handling to `WebStandardStreamableHTTPServerTransport`.

Test plan

• `pnpm --filter @modelcontextprotocol/server test`: 57 tests pass (up from 55)
• `pnpm --filter @modelcontextprotocol/server typecheck`: clean
• New in `test/server/streamableHttp.test.ts` inside the existing `enableJsonResponse` describe block: a regression test confirms `Accept: application/json` alone now returns 200 (previously 406); a second test confirms `Accept: text/event-stream` alone still returns 406.

Fixes #1944

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🦋 Changeset detected

Latest commit: caee461

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

This PR includes changesets to release 5 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/express	Patch
@modelcontextprotocol/fastify	Patch
@modelcontextprotocol/hono	Patch
@modelcontextprotocol/node	Patch

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