Fix outdated JSON schema output example in README

[Copilot]

The README's example JSON schema output for JsonSchema.FromType<Person>() no longer matched what the library actually generates, making the documentation misleading.

Changes

• FirstName/LastName: Added "minLength": 1 — now emitted due to [Required] attribute
• Gender: Removed redundant oneOf wrapper; now emits $ref directly for non-nullable value type references
• Company/Manufacturer: Fixed oneOf array ordering from [$ref, null] → [null, $ref]

Both JsonSchema.FromType<Person>() (System.Text.Json) and NewtonsoftJsonSchemaGenerator.FromType<Person>() now produce identical output for this class, matching the updated example.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Is this documentation still valid?</issue_title>
<issue_description>The JsonSchema class can be used as follows:

var schema = JsonSchema.FromType<Person>();
var schemaData = schema.ToJson();
var errors = schema.Validate("{...}");

foreach (var error in errors)
    Console.WriteLine(error.Path + ": " + error.Kind);

schema = await JsonSchema.FromJsonAsync(schemaData);
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Is this documentation still valid? #1587

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