Add background step deferral infrastructure and metadata plumbing

[lokesh755]

Summary

Adds the deferral mechanism and metadata plumbing needed for background step execution. Background steps buffer their outputs, environment variables, and path changes instead of writing to shared state directly — these are flushed to the job context at wait time on the main thread.

Changes

• ExecutionContext.cs — Adds DeferredOutputs, DeferredEnvironmentVariables, DeferredPrependPath, DeferOutcomeConclusion properties with corresponding FlushDeferred*() methods. Adds SetBackgroundStepMetadata() for timeline record metadata. Guards UpdateGlobalStepsContext behind DeferOutcomeConclusion flag.
• FileCommandManager.cs — GITHUB_OUTPUT, GITHUB_ENV, and GITHUB_PATH file commands check for deferred collections and buffer writes when running as a background step.
• JobServerQueue.cs — MergeTimelineRecords preserves background step metadata fields (IsBackground, BackgroundControlType, BackgroundControlStepIds, ParallelGroupId).

Related to https://github.com/github/c2c-actions/issues/9534

[Copilot]

Pull request overview

Adds infrastructure to support “background step” execution by deferring side effects (outputs, env vars, PATH changes, and outcome/conclusion propagation) and by plumbing background-step metadata through timeline record merging and step result reporting.

Changes:

• Introduces deferred collections and FlushDeferred*() APIs on IExecutionContext/ExecutionContext, and gates UpdateGlobalStepsContext() behind DeferOutcomeConclusion.
• Updates GITHUB_OUTPUT, GITHUB_ENV, and GITHUB_PATH file commands to buffer into deferred collections when present.
• Updates timeline record merge logic to preserve background-step metadata fields.

Show a summary per file

File	Description
src/Runner.Worker/FileCommandManager.cs	Buffers output/env/path file-command effects into deferred collections when running in a background-step context.
src/Runner.Worker/ExecutionContext.cs	Adds deferral properties and flush APIs; defers global steps context updates; attempts to emit background metadata in StepResult.
src/Runner.Common/JobServerQueue.cs	Preserves background-step metadata when merging multiple timeline record updates.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 3