bug: missing "global $relevant_chunks" declaration in hf_classifier retrieval flows

Describe the bug

In the hf classifier check retrieval flow (Colang), the masked/transformed $relevant_chunks value is reassigned locally without a global declaration, so the transformation does not propagate outside the flow's local scope. This affects:

• nemoguardrails/library/hf_classifier/flows.co (lines 24-25, flow hf classifier check retrieval $classifier)
• nemoguardrails/library/hf_classifier/flows.v1.co (matching retrieval subflow)

This is a pre-existing bug, unrelated to the RailOutcome migration in PR #2151, surfaced during review there.

Steps To Reproduce

1. Configure a rail using hf classifier check retrieval with a classifier that triggers a transform (is_transform=True) on $relevant_chunks.
2. Observe that $relevant_chunks = $response.transform_text["relevant_chunks"] is assigned without global $relevant_chunks.
3. Check downstream flow/action usage of $relevant_chunks after this subflow returns.

Expected Behavior

The transformed $relevant_chunks value should be visible to the calling flow/context after the subflow completes, since it's meant to update the shared retrieval context.

Actual Behavior

Because global $relevant_chunks is not declared before reassignment, the transform assignment stays scoped to the local flow and does not propagate to the caller/global context, so callers may still see the pre-transform value.

References

• PR: https://github.com/NVIDIA-NeMo/Guardrails/pull/2151
• Review comment: https://github.com/NVIDIA-NeMo/Guardrails/pull/2151#discussion_r3585336499
• Reported by: @Pouyanpi

Acceptance Criteria

• Add global $relevant_chunks before the reassignment in flows.co and flows.v1.co retrieval transform branches.
• Add/adjust a test confirming the transformed $relevant_chunks propagates to the caller context.