[Server] Answer a JSON POST with only its own responses

[vbcherepanov]

Fixes #467.

With PHP-FPM, requests of the same session run in parallel and put their responses into the same session queue. createJsonResponse() took the whole queue, so one request got all responses as a JSON array and the others got 202 with nothing.

Now the transport collects the ids from its own POST body and takes only those responses. The rest stays in the queue. Batches are still answered with an array, single messages with one object, and an invalid message without a usable id still gets its error.

Protocol::consumeOutgoingMessages() gets the ids as an optional second argument. Without it nothing changes, so the SSE path and StdioTransport work as before.

The reproducer is testConcurrentPostsSharingASessionEachReceiveTheirOwnResponse: the second POST is handled completely between the first one queueing its response and reading the queue. It fails on main.

Not part of this PR:

• the lost update on the session data itself ([Streamable HTTP][Server] Concurrent requests in the same session can overwrite queued responses and cause stale/unknown message IDs #275). The queue is read and written like before, and nothing is written if there is nothing to take.
• notifications queued outside of an SSE stream (e.g. resource subscriptions) are not added to a JSON response anymore, since they could only go there as an invalid array. They stay in the queue.