Plugin definitions not cached causing full annotation discovery on every request

Problem/Motivation

The ConditionalFieldsHandlersManager bypasses plugin cache by passing raw discovery to the factory (line 35):

$this->factory = new ContainerFactory($this->getDiscovery());

Every createInstance() triggers full annotation discovery instead of using cache. Impact scales with number of modules installed * number of conditional field dependencies.

Execution time:

ConditionalFieldsFormHelper::getState     → 1,276,279 μs (29.9%)
  └─ StaticDiscoveryDecorator::getDefinition × 132 calls → 1,252,207 μs (29.4%)
      └─ AnnotatedClassDiscovery::getDefinitions → 1,019,607 μs (23.9%)
          └─ file_exists × 30,856 calls → 831,358 μs (19.5%)

Steps to reproduce

1. Set up a node type with x fields and conditional dependencies
2. Observe uncached definitions called repeatedly

Proposed resolution

Remove line 35. Parent class DefaultPluginManager creates a properly cached factory automatically.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A