in_http: leftover pipelined data is never re-parsed after a request is consumed #12653

Bug Report

Describe the bug

After a request is consumed, the leftover bytes are moved to the front of the connection buffer and the parser is
re-initialised — but the remaining buffer is never parsed again. A pipelined request therefore sits unprocessed
until more data happens to arrive on the socket. If the client is waiting for the second response, it waits
until the keep-alive timeout.

This is separate from #12652 (which is about the body length being wrong): it reproduces with
Transfer-Encoding: chunked, where the request length is exact and the leftover bytes are correctly preserved.

To Reproduce

One connection, two pipelined chunked requests, then wait:

POST / HTTP/1.1
Host: x
Content-Type: application/json
Transfer-Encoding: chunked

7\r\n{"a":1}\r\n0\r\n\r\n
POST /2 HTTP/1.1
Host: x
Content-Type: application/json
Content-Length: 7

{"b":2}

Result: one response, one record ({"a"=>1}). The second request produces nothing.

Now send a third request on the same connection. The second one is then finally parsed — its record
({"b"=>2}) appears, tagged 2, before the third is handled. So the bytes were buffered correctly all along;
nothing re-ran the parser on them.

Root cause

plugins/in_http/http_conn.c, after MK_HTTP_PARSER_OK:

if (0 < (conn->buf_len - request_len)) {
    memmove(conn->buf_data, &conn->buf_data[request_len],
            conn->buf_len - request_len);
    conn->buf_data[conn->buf_len - request_len] = '\0';
    conn->buf_len -= request_len;
}
else { ... }

/* Reinitialize the parser ... */
memset(&conn->session.parser, 0, sizeof(struct mk_http_parser));
mk_http_parser_init(&conn->session.parser);
http_conn_request_init(&conn->session, &conn->request);

The parser is reset and the leftover data is in the buffer, but the function returns without calling
mk_http_parser() on it. Parsing only happens from http_conn_event(), which runs on a read event.

Expected behavior

After moving the leftover bytes down, the parser should be run again on them (loop, or re-enter the parse) so
pipelined requests are handled immediately.

Your Environment

• Fluent Bit 5.1.5 (fluent/fluent-bit:latest)
• Linux, aarch64, official container image
• Input: http plugin, default settings

Impact

Pre-auth. HTTP pipelining is effectively broken: the second and later requests on a connection are not answered
until unrelated data arrives, and are lost entirely if it never does. Clients that pipeline see stalls up to the
keep-alive timeout.