JSON Schema: Reference-count guided inlining and schema/schematic merge

[stephenberry]

JSON Schema: Reference-count guided inlining and schema/schematic merge

Addresses #2469

Overview

$defs entries that are referenced only once are now inlined at their $ref site. Multi-use entries remain in $defs as before.

Before (struct with a single Address field):

{
  "properties": {
    "address": { "$ref": "#/$defs/Address" }
  },
  "$defs": {
    "Address": { "type": "object", "properties": { ... } }
  }
}

After:

{
  "properties": {
    "address": { "type": "object", "properties": { ... } }
  }
}

When a type is referenced from multiple sites, it stays in $defs with $ref as before.

Implementation

The inlining operates on the typed schema tree before serialization — no JSON parsing/re-serialization round-trip.

1. count_schema_refs — walks the tree counting $ref string occurrences
2. try_inline_ref — for single-use $ref nodes, moves the definition from $defs into the reference site, merging any sibling metadata (e.g. title, description)
3. inline_single_use_refs — recursive traversal that applies inlining at every reference site (properties, items, additionalProperties, oneOf, prefixItems), with chained resolution for nested single-use types
4. prune_inlined_defs — removes consumed entries; drops $defs entirely when empty

Unified schema type

schema and detail::schematic are merged into a single schema struct. This eliminates the type mismatch that previously prevented typed inlining (properties stored schema but definitions were schematic).

• User-facing API unchanged — glz::json_schema<T> specializations continue to work with designated initializers (schema x{.description = "...", .minimum = 0})
• Structural fields (properties, items, oneOf, defs, etc.) are at the bottom of the struct, all std::optional defaulting to null
• items and additionalProperties use std::shared_ptr<schema> to break the recursive type cycle while keeping schema copyable
• Metadata (glz::meta<schema>) moved to an external specialization (required because the self-referential container members need the type to be complete)

Other changes

• merge_schema_attrs applies user json_schema metadata with merge semantics rather than wholesale assignment, preserving generated fields (e.g. a user-specified minimum no longer discards the type's maximum)
• make_primitive_schema returns std::shared_ptr<schema> (was schema) to match the new items/additionalProperties type
• openapi.hpp and http_server.hpp updated from detail::schematic to schema

[packit-as-a-service]

One of the tests failed for 0ee41e0. @admin check logs https://download.copr.fedorainfracloud.org/results/packit/stephenberry-glaze-2501/fedora-rawhide-s390x/10332576-glaze/builder-live.log, packit dashboard https://dashboard.packit.dev/jobs/copr/3475869 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/10332576/

[packit-as-a-service]

One of the tests failed for e7d688f. @admin check logs https://download.copr.fedorainfracloud.org/results/packit/stephenberry-glaze-2501/fedora-rawhide-s390x/10332583-glaze/builder-live.log, packit dashboard https://dashboard.packit.dev/jobs/copr/3475873 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/10332583/