feat: add support for helm 4 --server-side upgrade flag (#2641)

* feat: add support for helm 4 --server-side upgrade flag

Add support for the helm 4 upgrade flag --server-side which accepts
"true", "false", or "auto" (default "auto"). This allows users to
explicitly control server-side apply behavior, which is needed for
releases originally installed with Helm 3 and being managed with Helm 4.

The flag can be configured via:
- CLI: --server-side flag on sync, apply, and diff commands
- helmDefaults.serverSide in helmfile.yaml
- releases[].serverSide per-release override

Precedence: release-level > CLI flag > helmDefaults.
Errors are returned when serverSide is set but running Helm 3, or when
an invalid value is provided.

Closes #2640

Signed-off-by: yxxhero <aiopsclub@163.com>

* test: update TestGenerateID expected hashes for new ServerSide field

Adding ServerSide *string to ReleaseSpec changes spew's %#v output and
shifts the FNV hash used by generateValuesID. Update the hard-coded want
values to the new deterministic hashes.

Signed-off-by: yxxhero <aiopsclub@163.com>

---------

Signed-off-by: yxxhero <aiopsclub@163.com>

pkg/app/diff_test.go

@@ -47,6 +47,7 @@ type diffConfig struct {
 	reuseValues              bool
 	logger                   *zap.SugaredLogger
 	takeOwnership            bool
+	serverSide               string
 	enforceNeedsAreInstalled bool
 }
 
@@ -188,6 +189,9 @@ func (a diffConfig) SuppressOutputLineRegex() []string {
 func (a diffConfig) TakeOwnership() bool {
 	return a.takeOwnership
 }
+func (a diffConfig) ServerSide() string {
+	return a.serverSide
+}
 
 func (a diffConfig) EnforceNeedsAreInstalled() bool {
 	return a.enforceNeedsAreInstalled