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>

docs/configuration.md

@@ -433,6 +433,7 @@ The following `helmDefaults` fields are also available but not shown in the exam
 | `skipRefresh` | bool | false | Skip running `helm dependency up` |
 | `forceConflicts` | bool | false | Force server-side apply changes against conflicts (Helm 4 only) |
 | `takeOwnership` | bool | false | Take ownership of existing resources |
+| `serverSide` | string | | Controls the helm 4 `--server-side` flag. Must be `"true"`, `"false"`, or `"auto"` (Helm 4 only) |
 | `trackMode` | string | `""` | Default tracking mode for resources. See [Advanced Features](advanced-features.md#resource-tracking-with-kubedog) |
 | `disableAutoDetectedKubeVersionForDiff` | bool | false | Disable auto-detected kubeVersion being passed to helm diff |
 
@@ -456,6 +457,7 @@ The following per-release fields are also available:
 | `skipRefresh` | bool | false | Per-release skip for `helm dependency up` |
 | `disableAutoDetectedKubeVersionForDiff` | bool | false | Disable auto-detected kubeVersion for helm diff on this release |
 | `takeOwnership` | bool | false | Take ownership of existing resources for this release |
+| `serverSide` | string | | Controls the helm 4 `--server-side` flag for this release. Must be `"true"`, `"false"`, or `"auto"` (Helm 4 only) |
 | `forceConflicts` | bool | false | Force server-side apply against conflicts (Helm 4 only) |
 | `description` | string | | Description of the release |
 | `enableDNS` | bool | false | Enable DNS lookups when rendering templates |