fix: support OCI chart digest syntax (@sha256:...) (#2398)

fix: support OCI chart digest syntax in chart URLs and version fields

Helm supports pinning OCI chart images by digest (@sha256:...), version
tag (:version), or both (:version@sha256:digest) since helm/helm#12690.
Helmfile failed to parse these formats, incorrectly constructing helm
commands and losing version/digest information embedded in chart URLs.

Root causes:
- resolveOciChart() used last ":" to find version tag, but sha256:abc
  contains ":", so digest URLs were split incorrectly
- getOCIQualifiedChartName() included :version and @digest in chartName
  with no parsing of either source
- appendChartVersionFlags() passed release.Version verbatim to --version
  flag, including any digest suffix
- ChartPull() discarded the tag from resolveOciChart but did not
  preserve digest in the URL

This commit adds parseOCIChartRef() and parseVersionDigest() utilities,
then updates the OCI chart handling pipeline so that:
- Digests are preserved in the chart URL passed to helm pull
- Version tags are extracted cleanly for the --version flag
- Both chart URL and version field are parsed for version/digest info

Fixes #2097

Signed-off-by: Aditya Menon <amenon@canarytechnologies.com>

test/integration/run.sh

@@ -117,6 +117,7 @@ ${kubectl} create namespace ${test_ns} || fail "Could not create namespace ${tes
 . ${dir}/test-cases/issue-2281-array-merge.sh
 . ${dir}/test-cases/issue-2353-layer-array-replace.sh
 . ${dir}/test-cases/issue-2247.sh
+. ${dir}/test-cases/issue-2097.sh
 . ${dir}/test-cases/issue-2291.sh
 . ${dir}/test-cases/oci-parallel-pull.sh
 . ${dir}/test-cases/issue-2297-local-chart-transformers.sh