Allow more accept headers

[halter73]

The StreamableHttpHandler was a bit pickier about accept headers than I think it should be. While the spec does say the following:

The client MUST include an Accept header, listing both application/json and text/event-stream as supported content types.

https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#sending-messages-to-the-server

I don't think it's necessary to the server to be so strict enforcing this. If a client wants to say it accepts "*/*", it can deal with the fallout if it really cannot handle a JSON or SSE response. The most visible impact of this will probably be the response you see if you enter a Streamable HTTP endpoint URL directly into your browser's address bar since it does typically accept "*/*". Now, instead of seeing a 406 response with the following body:

{
  "error": {
    "code": -32000,
    "message": "Not Acceptable: Client must accept text/event-stream"
  },
  "id": "",
  "jsonrpc": "2.0"
}

You'll see a 404 response with this body instead:

{
  "error": {
    "code": -32001,
    "message": "Session not found"
  },
  "id": "",
  "jsonrpc": "2.0"
}

I think both of these are about just as useful to someone hitting a Streamable HTTP endpoint URL directly with the browser in conveying that something is responding to the endpoint, but a plain GET request from the browser without the right headers is unsupported.

We could decide to still reject "*/*" considering the spec says clients MUST list both application/json and text/event-stream, but it would take a little more code. By leveraging the MatchesMediaType() method, we support accept headers like "*/*" and headers with extra parameters like the "text/event-stream, application/json;q=0.9" from the test. Whatever we decide for "*/*" we should ignore these extra parameters.