Refactoring fixes

[mvSapphire]

Changes

Bug fixes

• PipelineStepFactory: Create and CreateCompensation now throw InvalidOperationException with the unregistered type name instead of silently returning null (which caused a NullReferenceException downstream)
• PipelineBuilder.Build(): Guard against duplicate FinishStep — calling Build() multiple times no longer corrupts the pipeline
• PipelineBuilder: Removed incorrect volatile modifier from _context (field is only assigned in the constructor)

API improvements

• IPipelineStepFactory: Removed IServiceProvider ServiceProvider property (Service Locator anti-pattern). Added ILoggerFactory GetLoggerFactory() instead. PipelineBuilder now uses this method to obtain the logger factory
• PipelineContext / PipelineBuilder / Pipeline: Dropped where TResult : class constraint, allowing value types and record structs as pipeline results. RunAsync returns default(TResult) instead of null for nested pipelines
• PipelineExecutionException / NestedPipelineExecutionException: Added standard .NET exception constructors (parameterless, (string), (string, Exception))

Documentation

• PipelineBuilder.Parallel() and ParallelStep: Added XML doc warning that parallel steps share the same context instance and must synchronize writes (e.g. Interlocked, lock, concurrent collections)
• IPipelineStep.NextStep: Documented that the setter is managed by pipeline infrastructure and should not be set by consumer code
• PipelineDiagramsService.GetDiagrams(): Documented that diagrams are cached after the first call with no invalidation

Logging

• PipelineDiagramsService.GetDiagrams(): Changed exception logging from LogDebug to LogError

Test cleanup

• TestParallelStep: Removed unused NextStep property