Prevent rollback stage builder panics on empty stage lists

[rootp1]

What this PR does:

Prevents rollback-enabled pipeline stage builders from panicking on empty stage lists in the Terraform, Kubernetes, Kubernetes multicluster, and Cloud Run piped v1 plugins.

Why we need it:

These builders currently call slices.MinFunc on an empty slice when rollback is enabled without configured stages, which crashes the builder instead of returning a controlled validation error.

Which issue(s) this PR fixes:

Fixes #7001

Summary

• guard rollback stage synthesis when no stages are configured
• return a stable validation error instead of panicking
• add regression coverage for the empty-stage rollback case in each affected plugin

Linked Issue

Fixes #7001

What Changed

• added empty-stage validation before computing rollback indices in the affected builders
• updated multicluster and Cloud Run callers to propagate builder errors
• extended existing table-driven tests to cover rollback-enabled empty stage lists

Verification

• go -C pkg/app/pipedv1/plugin/terraform test ./deployment -run 'TestPlugin_BuildPipelineSyncStages' && go -C pkg/app/pipedv1/plugin/kubernetes test ./deployment -run 'Test_buildPipelineStages' && go -C pkg/app/pipedv1/plugin/kubernetes_multicluster test ./deployment -run 'Test_buildPipelineStages' && go -C pkg/app/pipedv1/plugin/cloudrun test ./deployment -run 'Test_buildPipelineStages' — passed
• make build/go — passed
• make check/gen/code — passed
• make check — failed: yarn is not installed locally, so the repository-wide web build step in build/web could not run
• make test/integration — failed: Docker is unavailable (/var/run/docker.sock missing) for the Firestore and MySQL integration suites
• make check/dco — failed before the new commit because upstream master HEAD commit 25ae69937e55e8330bff96c34346718f3e5e7148 is missing a Signed-off-by line; this PR commit itself is signed off
• make test/go MODULES=pkg/app/pipedv1/plugin/terraform,pkg/app/pipedv1/plugin/kubernetes,pkg/app/pipedv1/plugin/kubernetes_multicluster,pkg/app/pipedv1/plugin/cloudrun — not completed locally: the broader module-wide run did not finish in a reasonable time after entering the heavier Kubernetes suites, so the focused regression command above was used as the closest confirmed local alternative

Scope

• Only the affected piped v1 rollback stage builders and their focused regression tests were changed.

Does this PR introduce a user-facing change?:

• How are users affected by this change: rollback-enabled empty pipeline requests now return a validation error instead of crashing the builder path
• Is this breaking change: No
• How to migrate (if breaking change): Not applicable