test histogram bucket boundaries

[GianIac]

If you'd like to help with Numax, this is a great place to start, thank you so much for taking a look !

Add tests for remote-op duration histogram bucket mapping in observability metrics.

Why

DurationHistogram uses fixed manual buckets.
Boundary tests reduce the chance of silent regressions in latency bucket accounting.

Scope

• in crates/nx-core/src/observability.rs tests:
  • verify bucket placement at exact upper bounds
  • verify placement just above a bound
  • verify sum and count accumulation consistency
• no metric naming/contract changes

Done when

• histogram boundary tests pass
• bucket, sum, and count behavior is explicitly asserted
• existing observability tests remain green

Notes

Please keep tests deterministic and narrowly scoped.

Thanks again for considering contributing to Numax.