[client] Handle interface lookup errors in iOS DNS index helper (#6999)

## Describe your changes

`getInterfaceIndex` in the iOS upstream DNS resolver dereferenced the
result of `net.InterfaceByName` before checking the error, so a missing
interface (e.g. during teardown or renaming) caused a nil-pointer panic
instead of a DNS client error.

The helper now returns a wrapped error before touching the interface;
the only caller, `GetClientPrivate`, already propagates the error.

The helper moved to an un-build-tagged file so it can be unit-tested on
host platforms while remaining available to the iOS build. Added a test
covering the missing-interface path. Verified with the new host test, an
iOS arm64 CGO compile, and `git diff --check`.

## Issue ticket number and link

N/A

## Stack

<!-- branch-stack -->

Standalone PR based on `main`.

### Checklist

- [x] Is it a bug fix
- [ ] Is a typo/documentation fix
- [ ] Is a feature enhancement
- [ ] It is a refactor
- [x] Created tests that fail without the change (if possible)
- [x] This change does **not** modify the public API, gRPC protocols,
functionality behavior, CLI / service flags, or introduce a new feature
— **OR** I have discussed it with the NetBird team beforehand (link the
issue / Slack thread in the description). See
[CONTRIBUTING.md](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTING.md#discuss-changes-with-the-netbird-team-first).

> By submitting this pull request, you confirm that you have read and
agree to the terms of the [Contributor License
Agreement](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTOR_LICENSE_AGREEMENT.md).

## Documentation

Select exactly one:

- [ ] I added/updated documentation for this change
- [x] Documentation is **not needed** for this change (explain why)

Internal crash fix in the iOS DNS path; no user-facing behavior or
configuration changes.

### Docs PR URL (required if "docs added" is checked)

N/A


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Improved handling of network interface lookup failures with clearer
error messages that identify the affected interface.
* Added validation for network interface lookups, including reliable
error handling when an interface cannot be found.
* **Tests**
* Added coverage for both successful interface resolution and
missing-interface scenarios.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

client/internal/dns/interface_index.go

@@ -0,0 +1,15 @@
+package dns
+
+import (
+	"fmt"
+	"net"
+)
+
+func getInterfaceIndex(interfaceName string) (int, error) {
+	iface, err := net.InterfaceByName(interfaceName)
+	if err != nil {
+		return 0, fmt.Errorf("lookup interface %q: %w", interfaceName, err)
+	}
+
+	return iface.Index, nil
+}

client/internal/dns/interface_index_test.go

@@ -0,0 +1,35 @@
+package dns
+
+import (
+	"net"
+	"testing"
+)
+
+func TestGetInterfaceIndexExisting(t *testing.T) {
+	interfaces, err := net.Interfaces()
+	if err != nil {
+		t.Fatalf("list network interfaces: %v", err)
+	}
+	if len(interfaces) == 0 {
+		t.Fatal("expected at least one network interface")
+	}
+
+	iface := interfaces[0]
+	index, err := getInterfaceIndex(iface.Name)
+	if err != nil {
+		t.Fatalf("look up existing interface %q: %v", iface.Name, err)
+	}
+	if index != iface.Index {
+		t.Fatalf("expected interface index %d, got %d", iface.Index, index)
+	}
+}
+
+func TestGetInterfaceIndexMissing(t *testing.T) {
+	index, err := getInterfaceIndex("netbird-interface-that-does-not-exist")
+	if index != 0 {
+		t.Fatalf("expected missing interface index to be 0, got %d", index)
+	}
+	if err == nil {
+		t.Fatal("expected missing interface lookup to return an error")
+	}
+}

client/internal/dns/upstream_ios.go

@@ -130,8 +130,3 @@ func GetClientPrivate(iface privateClientIface, upstreamIP netip.Addr, dialTimeo
 	}
 	return client, nil
 }
-
-func getInterfaceIndex(interfaceName string) (int, error) {
-	iface, err := net.InterfaceByName(interfaceName)
-	return iface.Index, err
-}