fix(owlbot-postprocessor): ensure GCR digest has sha256 prefix

[quartzmo]

This PR was dry-run tested with the following script locally:

#!/bin/bash
# Local dry-run verification script to test the cloudbuild.yaml bash logic.
# Execute this in your gMac terminal.

echo "=== 1. Testing GCR query for latest image ==="
PREV_DIGEST=$(gcloud container images list-tags gcr.io/cloud-devrel-public-resources/owlbot-ruby --filter="tags=latest" --format="value(digest)" 2>/dev/null || true)

if [ -z "$PREV_DIGEST" ]; then
  echo "Error: Could not retrieve the previous latest digest from GCR. Are you authenticated?"
  exit 1
fi

echo "Raw GCR query returned: '$PREV_DIGEST'"

echo "=== 2. Parsing Digest and formatting ==="
HEX_DIGEST=${PREV_DIGEST#sha256:}
FULL_PREV_DIGEST="sha256:$HEX_DIGEST"

echo "Parsed HEX_DIGEST: '$HEX_DIGEST'"
echo "Formatted FULL_PREV_DIGEST: '$FULL_PREV_DIGEST'"

echo "=== 3. Dry-running gcloud add-tag command ==="
echo "If this were live, it would execute:"
echo "  gcloud container images add-tag gcr.io/cloud-devrel-public-resources/owlbot-ruby@$FULL_PREV_DIGEST gcr.io/cloud-devrel-public-resources/owlbot-ruby:deprecated-public-image-$HEX_DIGEST --quiet"

echo "=== 4. Syntax validation ==="
if [[ "$FULL_PREV_DIGEST" =~ ^sha256:[a-f0-9]{12,64}$ ]]; then
  echo "Success: Digest format '$FULL_PREV_DIGEST' is valid."
else
  echo "Fail: Digest format '$FULL_PREV_DIGEST' is invalid. Must match sha256:<hex>."
  exit 1
fi