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/config/diff.go

@@ -53,6 +53,8 @@ type DiffOptions struct {
 	SkipSchemaValidation    bool
 	// TakeOwnership is true if the ownership should be taken
 	TakeOwnership bool
+	// ServerSide controls the helm 4 --server-side flag. Must be "true", "false", or "auto".
+	ServerSide string
 }
 
 // NewDiffOptions creates a new Apply
@@ -219,3 +221,8 @@ func (t *DiffImpl) SkipSchemaValidation() bool {
 func (t *DiffImpl) TakeOwnership() bool {
 	return t.DiffOptions.TakeOwnership
 }
+
+// ServerSide returns the ServerSide.
+func (t *DiffImpl) ServerSide() string {
+	return t.DiffOptions.ServerSide
+}