Batched requests' responses order is not respecting the order of requests

[aozherelyeva]

Describe the bug
When sending a message with the batch payload of several requests, like:

val payload = encodeMessages(listOf(firstRequest, secondRequest))

So the body is:

[{"id":"first","method":"tools/list"},{"id":"second","method":"resources/list"}]

the response from the server will be

[{"id":"second",...},{"id":"first",...}]

instead of

[{"id":"first",...},{"id":"second",...}]

To Reproduce
Steps to reproduce the behavior:

1. Checkout to the branch zarechneva/streamable-http-transport-tests OR edit the StreamableHttpServerTransportTest and add the following checks to the batched requests wait for all responses before replying test:

// Check responses' order
        assertEquals(listOf(firstRequest.id, secondRequest.id), responses.map { it.id })
        val firstMeta = (responses[0] as ListToolsResult).meta
        val secondMeta = (responses[1] as ListResourcesResult).meta
        assertEquals("first", firstMeta?.get("label")?.jsonPrimitive?.content)
        assertEquals("second", secondMeta?.get("label")?.jsonPrimitive?.content)

instead of a TODO.
2. Run io.modelcontextprotocol.kotlin.sdk.server.StreamableHttpServerTransportTest#batched requests wait for all responses before replying

Expected behavior
The order of responses should match the order of messages in the batch request.