fix: conflict detection for triggers/pipelines in MergeApplicationConfig, async context propagation, comment accuracy

[Copilot]

MergeApplicationConfig silently overwrote duplicate trigger and pipeline names across merged workflow files, and ConfigDir was inconsistently set from the first workflow file rather than the application config. The async step.workflow_call goroutine used context.Background(), decoupling it entirely from parent cancellation. A misleading comment in the server startup path misrepresented the actual build flow.

Changes

• Conflict detection (config/config.go): MergeApplicationConfig now tracks seen trigger and pipeline names (via seenTriggers/seenPipelines) and returns a descriptive error on collision — matching the existing module conflict behavior:

  application "my-app": trigger name conflict: trigger "http-in" is defined in both "api" and "events"
  application "my-app": pipeline name conflict: pipeline "process-order" is defined in both "orders" and "fulfillment"
  

• ConfigDir consistency (config/config.go): combined.ConfigDir is now pre-set from appCfg.ConfigDir before the merge loop; the per-workflow fallback only applies when the application config dir is unset.

• Async context propagation (module/pipeline_step_workflow_call.go): The fire-and-forget goroutine now derives its timeout context from the parent ctx instead of context.Background(), allowing cancellation signals to propagate:

  go func(parentCtx context.Context, data map[string]any) {
      asyncCtx, cancel := context.WithTimeout(parentCtx, s.timeout)
      defer cancel()
      _, _ = target.Execute(asyncCtx, data)
  }(ctx, triggerData)

• Comment fix (cmd/server/main.go): Removed the incorrect reference to BuildFromApplicationConfig; comment now accurately describes the MergeApplicationConfig → buildEngine → BuildFromConfig flow.

• Tests (engine_multi_config_test.go): Added TestBuildFromApplicationConfig_TriggerNameConflict and TestBuildFromApplicationConfig_PipelineNameConflict.

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