refactor: reduce PropertyInjector complexity and allocations (#4931)

- Cache PropertyInfo lookups via static ConcurrentDictionary to avoid
  repeated reflection calls in source-gen paths
- Extract InjectPropertiesFromPlanAsync and RecurseIntoPropertyValueAsync
  to reduce cyclomatic complexity of InjectPropertiesRecursiveAsync
- Extract CreateSourceGeneratedDataGeneratorMetadata to fix duplicate
  PropertyHelper.GetPropertyInfo call (was called twice per property)
- Extract RentVisitedDictionary/ReturnVisitedDictionary for pool ops
- Use PropertyInitializationContext factory methods instead of manual
  object initializers, reducing duplication across 4 call sites
- Replace foreach with indexed for loops to avoid enumerator allocations
- Simplify GetInitializedDataSourceAsync with null-coalescing operator
- Remove redundant condition checks where HasProperties already implies
  properties exist
- Convert ResolveAndCachePropertiesCoreAsync from async to synchronous
  Task return to avoid unnecessary state machine allocation

Closes #4897

Author: thomhurst