feat: add version-specific OpenAPI compilers

[DerManoMann]

Summary

Introduces CompilerInterface and three version-specific implementations that transform a Specification into an OpenAPI document array. This is the compilation stage of the spec pipeline:

Specification → Compiler → OpenAPI document (array)

Depends on zircote#2054.

Design

Version-aware compilers

Each OpenAPI version (3.0, 3.1, 3.2) has its own compiler that handles version-specific differences rather than branching inside a single serializer.

• OpenApi31Compiler — base implementation (JSON Schema 2020-12, full 3.1 feature set)
• OpenApi30Compiler — extends 3.1, overrides for the draft-04 JSON Schema subset:
  • type is always a string (not array)
  • nullable as separate boolean keyword
  • exclusiveMinimum/exclusiveMaximum are booleans (alongside minimum/maximum)
  • No $ref siblings (summary/description stripped)
  • No webhooks, no const, no examples array (only singular example)
  • License: no identifier field (only url)
• OpenApi32Compiler — extends 3.1, adds Tag summary/parent/kind and PathItem query

CompilerInterface

interface CompilerInterface
{
    public function getVersion(): string;
    public function supports(string $version): bool;
    public function compile(Specification $specification): array;
    public function validate(Specification $specification): array;
}

The compile step is a pure function (Specification in → array out), making it independently testable without the rest of the pipeline.

Compiler responsibilities (from classic processor mapping)

┌──────────────────────────────┬──────────────────────────────────────────────────────────┐
│      Classic Processor       │                   Compiler Equivalent                    │
├──────────────────────────────┼──────────────────────────────────────────────────────────┤
│ MergeIntoComponents          │ Groups schemas/parameters/responses etc. into components │
├──────────────────────────────┼──────────────────────────────────────────────────────────┤
│ BuildPaths                   │ Groups operations by resolved path                       │
├──────────────────────────────┼──────────────────────────────────────────────────────────┤
│ AugmentSchemas (allOf merge) │ Emits allOf when both properties + allOf exist           │
└──────────────────────────────┴──────────────────────────────────────────────────────────┘

Inheritance strategy

3.0 and 3.2 extend the 3.1 compiler, overriding only the methods that differ. This keeps version-specific logic minimal and co-located rather than scattered across conditionals.

[DerManoMann]

Moved to zircote#2058 (base branch updated to master now that spec-dto is merged)