Keep the Step SecurityContext when the StepAction has none

[arpitjain099]

Changes

resolveStepRef merges a resolved StepAction into the Step that referenced it, and every field is guarded so the StepAction only wins where it actually set something. SecurityContext is the exception:

resolvedStep.SecurityContext = stepFromStepAction.SecurityContext

A StepAction that leaves securityContext unset therefore overwrites the Step's with nil. StepActionSpec.SetDefaults only walks Params and Results, so nothing fills it in before the merge and the nil really does reach the assignment.

This guards it the same way Command, Args, Script, Env, VolumeMounts and Results already are.

Fixes #10634

/kind bug
/area api

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Existing coverage kept the "StepAction sets a securityContext" direction (step-action-with-security-context-overwritten); the new case is its sibling for the unset direction, with runAsUser, runAsNonRoot and readOnlyRootFilesystem on the Step. Without the guard it fails:

- SecurityContext: s"&SecurityContext{...RunAsUser:*1001,RunAsNonRoot:*true,ReadOnlyRootFilesystem:*true,...}"
+ SecurityContext: nil

go build ./..., go vet and go test ./pkg/reconciler/taskrun/... are clean.

Release Notes

Fixed a bug where referencing a StepAction that does not set `securityContext` cleared the `securityContext` set on the Step.

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