Improvement: Increase unit test coverage to 90% line + branch: `score/TimeSlave/`

### What

Reach 90% line coverage and 90% branch coverage for the `score/TimeSlave/`
component once its implementation is available (see epic
eclipse-score/score#2691).

Steps:
1. Generate a per-component coverage report locally:
```
bazel coverage --config=time-x86_64-linux //score/TimeSlave/...
genhtml --branch-coverage bazel-out/_coverage/_coverage_report.dat -o coverage-report/
```
2. Identify files and branches below 90%
3. Add missing unit tests — focus areas:
- IPC channel (libTSClient) — connection, disconnection, error paths
- PTP sync loop — synchronization state transitions
- `clock_adjtime` / `clock_settime` integration — stub/mock the syscall
- TimeSlave process lifecycle — start, stop, restart

### References

- TimeSlave implementation epic: eclipse-score/score#2691
- S-CORE process (Coverage section):
https://github.com/eclipse-score/score/blob/main/docs/contribute/development/cpp/code_analysis.rst

### Depends on

TimeSlave implementation (eclipse-score/score#2691) must be sufficiently
complete before this task can begin.

### Estimates for realization

M - Week

### Category

- [ ] Affects Detailed Design

### Requirements / Architecture

- [x] Requirements / Architecture are not affected by this change?