Remove the "instance" field in LinkML validation errors

[candleindark]

This PR removes the "instance" field in each LinkML validation error in serialization, and doing so in a way that the correct schema for the serialization is readily available.

For example, the printout of the following code snippet produces the subsequent schema in which TrimmedValidationResult has no "instance" property.

import json

from dandisets_linkml_status_tools.cli.models import linkml_validation_errs_adapter

print(
    json.dumps(
        linkml_validation_errs_adapter.json_schema(mode="serialization"), indent=2
    )
)

{
  "$defs": {
    "Severity": {
      "description": "Enum to represent the severity of a validation message.",
      "enum": [
        "FATAL",
        "ERROR",
        "WARN",
        "INFO"
      ],
      "title": "Severity",
      "type": "string"
    },
    "TrimmedValidationResult": {
      "properties": {
        "type": {
          "title": "Type",
          "type": "string"
        },
        "severity": {
          "$ref": "#/$defs/Severity"
        },
        "message": {
          "title": "Message",
          "type": "string"
        },
        "instance_index": {
          "anyOf": [
            {
              "type": "integer"
            },
            {
              "type": "null"
            }
          ],
          "title": "Instance Index"
        },
        "instantiates": {
          "anyOf": [
            {
              "type": "string"
            },
            {
              "type": "null"
            }
          ],
          "title": "Instantiates"
        },
        "context": {
          "items": {
            "type": "string"
          },
          "title": "Context",
          "type": "array"
        }
      },
      "required": [
        "type",
        "severity",
        "message",
        "instance_index",
        "instantiates",
        "context"
      ],
      "title": "TrimmedValidationResult",
      "type": "object"
    }
  },
  "items": {
    "$ref": "#/$defs/TrimmedValidationResult"
  },
  "type": "array"
}