Add thread-safety locks to StepsContext

[lokesh755]

Summary

Adds thread-safety primitives to shared mutable state in the runner worker, preparing for concurrent step execution in background steps.

Why

Today, steps execute sequentially so these collections are never accessed concurrently. Background steps will allow multiple steps to run in parallel within a single job, meaning these shared collections can be mutated from multiple threads simultaneously. Adding locks now is a safe, no-op change for sequential execution and unblocks the background steps feature.

[Copilot]

Pull request overview

This PR prepares the runner for future concurrent/background step execution by extending step/timeline wire contracts with background-step metadata and introducing synchronization around some shared mutable state (e.g., telemetry/step context).

Changes:

• Extend Results/RSWebApi/DTWebApi step & timeline contracts to carry background-step metadata (e.g., is_background, control type, control step IDs, parallel group).
• Introduce a new pipeline step type (BackgroundStepControl) and update step deserialization to recognize it.
• Add locking around StepsContext mutations and some GlobalContext collection mutations to reduce concurrency hazards.

Show a summary per file

File	Description
src/Sdk/WebApi/WebApi/ResultsHttpClient.cs	Populates new background-step fields when converting timeline records to Results “Step” payloads.
src/Sdk/WebApi/WebApi/Contracts.cs	Adds background-step fields to Results Step contract.
src/Sdk/RSWebApi/Contracts/StepResult.cs	Adds background-step fields to RSWebApi step result contract.
src/Sdk/DTWebApi/WebApi/TimelineRecord.cs	Adds background-step fields to timeline records and copies them during clone.
src/Sdk/DTPipelines/Pipelines/StepConverter.cs	Updates step JSON deserialization to instantiate BackgroundStepControl.
src/Sdk/DTPipelines/Pipelines/Step.cs	Registers new step type via KnownType and enum value.
src/Sdk/DTPipelines/Pipelines/JobStep.cs	Adds/clones ParallelGroupId for job steps.
src/Sdk/DTPipelines/Pipelines/BackgroundStepControl.cs	Introduces new pipeline step type for background control flow (wait/wait-all/cancel).
src/Sdk/DTPipelines/Pipelines/ActionStep.cs	Adds/clones Background flag on action steps.
src/Runner.Worker/StepsContext.cs	Adds a lock around several StepsContext mutation paths.
src/Runner.Worker/PipelineTemplateEvaluatorWrapper.cs	Locks around telemetry list writes for mismatch reporting.
src/Runner.Worker/Handlers/HandlerFactory.cs	Locks around global HashSet mutations that track Node migration telemetry.
src/Runner.Worker/GlobalContext.cs	Adds a shared lock object intended for guarding global collections.
src/Runner.Worker/ActionManifestManagerWrapper.cs	Locks around telemetry list writes for mismatch reporting.
src/Runner.Worker/ActionManager.cs	Locks around JobTelemetry additions in action download path.
src/Runner.Worker/ActionCommandManager.cs	Locks around JobTelemetry additions for action command telemetry.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 6