Cleaner JSON schema with inline bool and string types

[stephenberry]

Inline JSON Schema for bool, string, and optional variants

Resolves #2473

Summary

Primitive types (bool, std::string, std::string_view, char) and their std::optional wrappers are now inlined directly in JSON Schema properties instead of being placed in $defs and referenced via $ref.

Before:

{
  "properties": {
    "name": { "$ref": "#/$defs/std::string" },
    "flag": { "$ref": "#/$defs/bool" }
  },
  "$defs": {
    "std::string": { "type": "string" },
    "bool": { "type": "boolean" }
  }
}

After:

{
  "properties": {
    "name": { "type": "string" },
    "flag": { "type": "boolean" }
  }
}

Nullable variants inline with the "null" type added:

{
  "nickname": { "type": ["string", "null"] }
}

Per-field metadata from json_schema is preserved alongside the inlined type:

{
  "name": { "type": "string", "description": "A name for something" }
}

Changes

• include/glaze/json/schema.hpp: Added a type field to glz::schema so properties can express {"type":"..."} directly. Modified the object handler to inline bool, str_t/char_t, and nullable_t wrappers of those types instead of creating $defs/$ref entries. If a user explicitly sets $ref via json_schema metadata, that is still respected.
• Tests: Updated expected schema strings across 6 test files to reflect the new inlined output.