Improve zbus test coverage

[rodrigopex]

Is your enhancement proposal related to a problem? Please describe.

File: subsys/zbus/zbus.c
Date: 2025-05-26 09:01:07
Exec Total Coverage
Lines: 287 296 97.0%
Functions: 23 24 95.8%
Branches: 205 216 94.9%

File: subsys/zbus/zbus_iterable_sections.c
Date: 2025-05-26 09:01:07
Exec Total Coverage
Lines: 20 20 100.0%
Functions: 4 4 100.0%
Branches: 22 24 91.7%

File: subsys/zbus/zbus_runtime_observers.c
Date: 2025-05-26 09:01:07
Exec Total Coverage
Lines: 71 79 89.9%
Functions: 6 8 75.0%
Branches: 57 62 91.9%

Describe the solution you'd like

Enforce 100% test coverage. One way to do this is to enable the uncovered features in the sample.yml and testcase.yml files.