Fix null type in explain endpoint

[bilalebi]

Description

Added a @model_validator so the type field is now derived from organism.strainType strain before model parsing, ensuring downstream fields receive the synthesized object only when present.
Removed the manual type injection from GenomeDetails.__init__, letting the validator handle it consistently for all subclasses.

Related JIRA Issue(s)

https://embl.atlassian.net/browse/ENSPLAT-272

Example(s)

Before

{
  "genome_id": "8bce37f6-5353-4fb4-962f-f7e9a6c4303d",
  ...
  "type": null,   <--- Boooh!
  ...
  "latest_genome": {
    "genome_id": "ce9c01fe-1d5c-4fea-a96a-b8eb3c3586f4",
    ...
    "type": null,   <--- Boooh!
    ...
  }
}

After

{
  "genome_id": "8bce37f6-5353-4fb4-962f-f7e9a6c4303d",
  ...
  "type": {             <-- Fixed!
    "kind": "strain",
    "value": "C57BL/6J"
  },
  "latest_genome": {
    "genome_id": "ce9c01fe-1d5c-4fea-a96a-b8eb3c3586f4",
    ...
    "type": {             <-- Fixed!
      "kind": "strain",
      "value": "C57BL/6J"
    },
    ...
  }
}