Add extensible task scheduler middleware pipeline

[dombrovsky]

Summary

• introduce an immutable enqueue, execution, and completion middleware pipeline over existing ITaskScheduler implementations
• add forward-scoped annotations, separate caller and producer cancellation, and per-registration operation-local state
• migrate interception, error handling, timeout, throttling, cancellation, and Microsoft logging extensions to middleware
• preserve terminal scheduler ownership and compatibility while documenting ordering, failure, cancellation, and wrapper-boundary semantics
• add focused pipeline and migrated-extension coverage across supported execution models

Why

TaskFlow decorators previously relied on private wrapper-specific transport and could not be composed through a common public extension point. The middleware pipeline provides a small public seam for custom policies without changing ITaskScheduler.Enqueue or requiring terminal schedulers to understand middleware.

Validation

• Release build passed for netstandard2.0, net8.0, and net10.0
• core TaskFlow tests passed: 230 on .NET 8 and 230 on .NET 10
• dependency-injection tests passed: 15 on each target
• Microsoft logging tests passed: 5 on each target

Closes #24