Refresh the shared stack-pin branch before applying releases

[sbaum1994]

Description

The stack pin bump workflow audits the self-managed stack on the latest default branch, then switches to the long-lived chore/stack-pin-bumps branch before applying a chart release.

When that shared branch predates new release metadata, the audit passes on main but the bump fails on the stale branch. The invocation chart v1.6.0 release exposed this mismatch: the resolver reported that deploy/helm/http-invocation was missing even though the entry had already merged to main.

Definition of Done

• Refresh an existing stack-pin branch onto the current default branch before applying a release bump.
• Preserve all previously accumulated pin commits.
• Start a missing stack-pin branch from the current default branch.
• Keep the refresh linear and fail loudly on conflicts.
• Add regression coverage for both existing and missing branch cases.
• Rerun the invocation chart v1.6.0 bump and confirm fix(stack): bump self-managed stack chart pins #1381 gains the invocation pin.

Resources

• Failed stack pin workflow: https://github.com/NVIDIA/nvcf/actions/runs/33364143571
• Invocation release metadata fix: ci(invocation): register Helm chart release #1386
• Accumulated stack pin pull request: fix(stack): bump self-managed stack chart pins #1381

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