Background steps execution engine

[lokesh755]

Summary

Adds the core background step execution engine to the runner, enabling concurrent step execution within a single job. Background steps (background: true) run asynchronously while the foreground step loop continues. Control-flow steps (wait, wait-all, cancel) coordinate synchronization and lifecycle management.

Changes

New files

• BackgroundStepCoordinator.cs — Coordinates background step execution, waiting, cancellation, and deferred state flushing. Manages a concurrency semaphore (default max 10).
• ControlFlowStepData.cs — Data class for wait/wait-all/cancel steps.
• BackgroundStepsL0.cs — Unit tests for background step coordination, waiting, cancellation, and failure propagation.

Modified files

• StepsRunner.cs — Integrates BackgroundStepCoordinator; background steps are queued without blocking the main loop; control-flow steps are dispatched directly; implicit wait-all runs before post-hooks as a safety net.
• JobExtension.cs — Initializes control-flow steps from BackgroundStepControl pipeline type; maps logical step IDs to external IDs for UI reference; creates implicit wait-all for uncovered background steps.
• ExecutionContext.cs — Adds background step metadata as optional parameters to CreateChild

Key design decisions

• Runner owns concurrency — run-service remains agnostic to step execution order
• Non-blocking semaphore — foreground steps execute immediately even when all background slots are occupied
• Implicit wait-all — safety net before post-hooks waits for any uncovered background steps

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