Improvement: Raise CI coverage threshold to 90% + enable branch coverage

### What

After Tasks
- https://github.com/eclipse-score/inc_time/issues/44
- https://github.com/eclipse-score/inc_time/issues/45
- https://github.com/eclipse-score/inc_time/issues/46

are complete and all components reach 90% line +
90% branch coverage, enforce the new threshold in the CI pipeline.

Update `.github/workflows/code_coverage.yml`:
```yaml
# Before:
min-coverage: 85

# After:
min-coverage: 90
genhtml-extra-flags: "--branch-coverage"
```

### Why

Raising the threshold without first reaching it (Tasks 5a–5c) would break
all PRs. This task is the final enforcement step — it gates the CI against
regressions below 90%.

### References

- Current coverage workflow:
https://github.com/eclipse-score/inc_time/blob/main/.github/workflows/code_coverage.yml
- Reusable workflow parameters (`min-coverage`, `genhtml-extra-flags`):
https://github.com/eclipse-score/cicd-workflows/blob/main/.github/workflows/cpp-coverage.yml

### Estimates for realization

S - Day

### Category

- [ ] Affects Detailed Design

### Requirements / Architecture

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