After comments and extraneous commas have been removed - kea-dhcp4 server started working as expected.

Final working config:

{
    "Dhcp4": {
        "interfaces-config": {
            "interfaces": [ "eno1" ],
            "dhcp-socket-type": "udp"
        },
  "lease-database": {
            "type": "memfile",
            "lfc-interval": 3600
  },
        "valid-lifetime": 4000,
        "renew-timer": 1000,
        "rebind-timer": 2000,
        "subnet4": [{
           "pools": [ { "pool": "192.168.1.3-192.168.1.254" } ],
           "subnet": "192.168.1.0/24",
           "id": 1
        }],
        "option-data": [
            {
                "name": "routers",
                "data": "192.168.1.1"
            },
            {
                "name": "domain-name-servers",
                "data": "192.168.1.1"
            }
        ],
       "loggers": [{
            "name": "*",
            "severity": "DEBUG"
        }]
    }
}

As Sam Forbis suggested in the comments - after comments and extraneous commas have been removed - kea-dhcp4 server started working as expected.

Final working config:

{
    "Dhcp4": {
        "interfaces-config": {
            "interfaces": [ "eno1" ],
            "dhcp-socket-type": "udp"
        },
  "lease-database": {
            "type": "memfile",
            "lfc-interval": 3600
  },
        "valid-lifetime": 4000,
        "renew-timer": 1000,
        "rebind-timer": 2000,
        "subnet4": [{
           "pools": [ { "pool": "192.168.1.3-192.168.1.254" } ],
           "subnet": "192.168.1.0/24",
           "id": 1
        }],
        "option-data": [
            {
                "name": "routers",
                "data": "192.168.1.1"
            },
            {
                "name": "domain-name-servers",
                "data": "192.168.1.1"
            }
        ],
       "loggers": [{
            "name": "*",
            "severity": "DEBUG"
        }]
    }
}