`Bug: unit_tests.dockerfile uses Go 1.19 but module requires Go 1.24`

## 📜 Description

`unit_tests.dockerfile` builds the unit test image with `FROM golang:1.19`, but `go.mod` specifies `go 1.24.0`. Go toolchain version enforcement means Go 1.19 will refuse to build a module requiring 1.24. The CI `build-test-kubeslice-controller` workflow uses this file and will fail.

## 👟 Reproduction steps

1. Run `docker build -f unit_tests.dockerfile .`
2. Build fails: `note: module requires Go 1.24`

## 👍 Expected behavior

The dockerfile should use the same Go version as declared in `go.mod`.

## 👎 Actual Behavior

Build fails due to Go version mismatch.

## ✅ Proposed Solution

Update `unit_tests.dockerfile` line 1 to:
```dockerfile
FROM golang:1.24 as builder
```

## 👀 Have you spent some time to check if this issue has been raised before?
- [x] I checked and didn't find any similar issue

## Code of Conduct
- [x] I agree to follow this project's Code of Conduct

Start with unit_tests.dockerfile line 1 and compare its image version with go.mod; inspect the build-test-kubeslice-controller workflow. Update the image version to match go.mod, then run docker build -f unit_tests.dockerfile . and confirm that the unit-test image builds.

Written by the indexing model from the issue text.