fix(monitoring): report actual process thread_count in system metrics

[SH20RAJ]

Fixes incorrect thread_count calculation in mofa-monitoring system metrics collection.

What changed:

• replaced Option::iter().count() misuse with correct inner task-set length handling
• added helper function for explicit and safe conversion to u32
• added regression tests for None and populated task-set cases

Why this aligns with GSoC idea:

• directly improves observability correctness under the Cognitive Observatory direction (ideas-list Idea 2 / mofa-monitoring development)

Closes #1201

[SH20RAJ]

Hi! Fixed the thread count calculation in mofa-monitoring. This ensures that system metrics accurately reflect the number of active threads in the process, which is essential for the 'Cognitive Observatory' monitoring goals (Idea 2). (Fixes #1201)

[SH20RAJ]

Addressing Issue #1201. This PR fixes an incorrect thread_count calculation in mofa-monitoring by correctly handling the task-set length instead of misusing Option::iter().count(). This ensures accurate system metrics in the 'Cognitive Observatory' (GSoC Idea #2), which is vital for production monitoring and performance tuning. 📊

[SH20RAJ]

Checks are green on this PR now. It is a small mofa-monitoring correctness fix for thread_count, which felt like a practical fit for the Cognitive Observatory / monitoring track in the GSoC 2026 ideas list. Happy to adjust naming or metric-contract details if maintainers would prefer a different shape here.