[comp log_backend] Unit Test completion

## What

Complete the Unit Test (`wp__verification_sw_unit_test`) for the component log_backend: add a formal `document::` with `:realizes: wp__verification_sw_unit_test` to existing unit test files. Unit testing verifies the detailed design and is integrated in CI/Build.

## How

**Process step:** [wf__verification_unit_test](https://eclipse-score.github.io/process_description/main/process_areas/verification/verification_workflows.html#wf__verification_unit_test)
Existing artifact: [[score/mw/log/](logging/score/mw/log/)]([score/mw/log/](logging/score/mw/log/))

## Definition of Done

All criteria of the [Verification Guideline](https://eclipse-score.github.io/process_description/main/process_areas/verification/guidance/verification_guideline.html) are met.

## Template & Folder Structure

- **Template:** [Module Template – Overview](https://eclipse-score.github.io/module_template/main/index.html)
- **Folder:** `score//src/` – see [Module Folder Structure](https://eclipse-score.github.io/module_template/main/index.html#module-layout)

*Please apply the linked template and folder structure to ensure consistency across the S-CORE project.*

Start by inspecting the existing unit test files under logging/score/mw/log/ and the score//src/ folder structure. Read the linked verification workflow, guideline, and Module Template, then add the required formal document:: entries so the unit test artifact realizes wp__verification_sw_unit_test and satisfies the guideline and CI/build expectations.

Written by the indexing model from the issue text.