fix: treat terragrunt run --all plan as hasPlanStep to skip single-file terraform show

[ramkalapala]

Fixes #2552

Problem

When using terragrunt run --all plan via a custom run step in the
workflow, driftctl scan completes successfully but then fails with:
error running terraform show: exit status 1

Root Cause

hasPlanStep in libs/execution/execution.go is only set to true
when a step has action == "plan":

hasPlanStep := lo.ContainsBy(planSteps, func(step scheduler.Step) bool {
    return step.Action == "plan"
})

With a custom run step, hasPlanStep stays false. After the run
step completes successfully, Digger falls into the !hasPlanStep branch
and calls terraform show against LocalPlanFilePath() — expecting a
single .tfplan file. But terragrunt run --all plan writes per-module
plan files across subdirectories, not a single file at that path.

Fix

Extend the hasPlanStep check to also treat run steps containing
run --all plan as a plan step, skipping the single-file
terraform show post-processing:

hasPlanStep := lo.ContainsBy(planSteps, func(step scheduler.Step) bool {
    return step.Action == "plan" ||
        (step.Action == "run" && strings.Contains(step.Value, "run --all plan"))
})

No new imports required — strings is already imported.

Testing

Existing test suite unchanged. Fix only affects workflows using
terragrunt run --all plan via a custom run step.

[ramkalapala]

Hi team, raised this PR to address #2552, @s1ntaxe770r, would really appreciate a review and approval when you get a chance.