Add appendServerSideFlagsForDiff to validate helm-diff plugin version
(v3.15.10+) before passing the --server-side flag to helm diff upgrade.
Extract resolveServerSideValue helper to share precedence logic between
upgrade and diff paths. Bump helm-diff recommended version to v3.15.10
across Dockerfiles, CI, and integration scripts.
Signed-off-by: yxxhero <aiopsclub@163.com>