Fix/commit certificate telemetry duration

[evrendag]

Summary

Fixes block processing telemetry in CommitCertificate so time.Since(start) is evaluated when the function actually returns, rather than when the defer statement is registered.

Problem

Passing time.Since(start) directly as an argument to a deferred call evaluates the duration immediately. As a result, the reported block processing time does not include the remaining commit work.

Fix

Wrap UpdateTelemetry in a deferred closure, matching the behavior already used by CommitCertificateParallel.

This ensures telemetry records the full commit duration, including the work performed after the defer is registered.

Scope

• No consensus behavior changes
• No state transition changes
• Telemetry-only fix
• Keeps serial and parallel commit paths consistent

This supersedes #557, which targeted main; this PR targets the required development branch per the contribution guidelines.