chore: switch to llvm-cov and nextest

[drmorr0]

• I certify that this PR does not contain any code that has been generated with GitHub Copilot or any other AI-based code generation tool, in accordance with this project's policies.

Description

• switch from grcov to cargo-llvm-cov
• start using cargo-nextest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.18%. Comparing base (f818d88) to head (ace04cd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   75.77%   70.18%   -5.60%     
==========================================
  Files          57       57              
  Lines        3208     3193      -15     
  Branches        0     3193    +3193     
==========================================
- Hits         2431     2241     -190     
- Misses        777      785       +8     
- Partials        0      167     +167     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.