Add containerImageValidation experimental feature flag that gates the
new container image validation behind an opt-in toggle. When the flag
is off (default), the legacy converter logic is used. When enabled,
the improved validation with expression handling runs.
The legacy code is duplicated to keep code paths fully isolated and
make the eventual cleanup diff minimal — just delete the legacy
functions and the flag guards.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
