Add #[When] attribute for contextual binding via attributes

[philipobenito]

Summary

Add a #[When(ConsumerClass::class)] parameter-level attribute as the attribute-surface counterpart to addContextualArgument().

This allows contextual binding to be declared at the class level rather than in the container bootstrap:

class FileCache implements CacheInterface {}

class LogService
{
    public function __construct(
        #[When(LogService::class)] CacheInterface $cache
    ) {}
}

Context

Contextual binding was introduced in 6.0 via addContextualArgument() on definitions. The #[When] attribute would provide the same capability through PHP attributes, consistent with the existing #[Inject] and #[Resolve] attributes.

The committee recommended deferring this to 6.1 to let the core contextual binding API settle before adding the attribute form.

Implementation notes

• Parameter-level attribute following the #[Inject]/#[Resolve] pattern
• May need a new interface (e.g. ContextualAttributeInterface) since #[When] declares a binding rule rather than directly resolving a value
• DefinitionAnalyser already handles attribute resolution during autowiring (lines 222-239 of DefinitionAnalyser.php), so compilation support is achievable
• The ArgumentReflectorTrait::resolveArgumentFromAttribute() method would need to handle the new attribute type

Target

6.1 minor release