Add CompositeMapperCompilerProvider for typed provider graph traversal

[janedbal]

Motivation

Consumers that pre-generate mappers (e.g. a Symfony command compiling input + output mappers for every *Input class and everything reachable from it) need to walk a provider graph to discover all transitively referenced classes — every reachable MapDelegate. Until now that required reflecting over provider internals (get_mangled_object_vars() over a plain object), which static analysis cannot attribute to any class: shipmonk/dead-code-detector reports it as an unknown read over unknown type and it degrades dead-code analysis of the whole consumer project.

What this adds

• CompositeMapperCompilerProvider — implemented by providers composed of other providers, exposing them via getInnerMapperCompilerProviders(): list<InputMapperCompilerProvider|OutputMapperCompilerProvider>. Implemented on all 12 composite attributes; notably MapArrayShape exposes the mappers wrapped in ArrayShapeItemMapping and MapObject exposes both constructor-arg and property providers.
• MapperCompilerProviderUtils::iterate() — yields a provider and all transitively nested providers, using only the interface. Consumers can then e.g. filter for MapDelegate without any reflection.

Completeness is enforced mechanically

Two tests keep implementations honest:

1. testTypedTraversalMatchesReflectiveTraversal compares the typed traversal against a reflective property walk over a graph exercising every attribute provider — forgetting a nested provider in any implementation fails the test naming the missed provider.
2. testProviderGraphContainsAllAttributeProviders globs src/Compiler/Attribute and forces every current and future provider attribute into that test graph — a new composite attribute cannot ship without exposing its inner providers.

Direction / follow-up

This is intended as a building block: input-mapper should ideally support mapper pre-generation natively. A follow-up could add e.g. a MapperPregenerator that takes root classes, expands MapDelegate dependencies via this traversal, and compiles input + output mappers for all of them — so consumer applications just call it from their console command instead of owning the discovery logic.

Backward compatible: purely additive, custom providers opt in by implementing the interface.

Co-Authored-By: Claude Code

Drafted by Claude