fix section schema

[koonpeng]

Bug fix

Fixed bug

schemars generates schemas with additionalProperties: false for enums.
When the enum is flatten, that additionalProperties: false is inherited by the parent
struct, which leads to schemas that can never be valid.

Example:

{
  "description": "Connect the request to a registered section.\n\n``` # bevy_impulse::Diagram::from_json_str(r#\" { \"version\": \"0.1.0\", \"start\": \"section_op\", \"ops\": { \"section_op\": { \"type\": \"section\", \"builder\": \"my_section_builder\", \"connect\": { \"my_section_output\": { \"builtin\": \"terminate\" } } } } } # \"#)?; # Ok::<_, serde_json::Error>(()) ```\n\nCustom sections can also be created via templates ``` # bevy_impulse::Diagram::from_json_str(r#\" { \"version\": \"0.1.0\", \"templates\": { \"my_template\": { \"inputs\": [\"section_input\"], \"outputs\": [\"section_output\"], \"buffers\": [], \"ops\": { \"section_input\": { \"type\": \"node\", \"builder\": \"my_node\", \"next\": \"section_output\" } } } }, \"start\": \"section_op\", \"ops\": { \"section_op\": { \"type\": \"section\", \"template\": \"my_template\", \"connect\": { \"section_output\": { \"builtin\": \"terminate\" } } } } } # \"#)?; # Ok::<_, serde_json::Error>(()) ```",
  "type": "object",
  "oneOf": [
    {
      "type": "object",
      "required": [
        "builder"
      ],
      "properties": {
        "builder": {
          "type": "string"
        }
      },
      "additionalProperties": false
    },
    {
      "type": "object",
      "required": [
        "template"
      ],
      "properties": {
        "template": {
          "type": "string"
        }
      },
      "additionalProperties": false
    }
  ],
  "required": [
    "type"
  ],
  "properties": {
    "config": {
      "default": null
    },
    "connect": {
      "default": {},
      "type": "object",
      "additionalProperties": {
        "$ref": "#/definitions/NextOperation"
      }
    },
    "type": {
      "type": "string",
      "enum": [
        "section"
      ]
    }
  }
},

Here the section schema needs to have a builder or template with no additional properties.
Which includes other properties like type, config etc, but type is also required which
breaks the schema.

Fix applied

Remove additionalProperties from the flattened field. The new unevaluatedProperties option in new json schema drafts addresses this issue, but schemars uses draft-07 so we can't use it. We could update the schema to draft-2020-12 but it's better not to make potentially breaking change.

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