chore: separate benchmarks into a distinct CI job

[maxrjones]

This PR changes the CI workflows such that the benchmarking test suite is not run on every environment in the matrix.

What changed

• Add --ignore tests/benchmarks to run-coverage and run-coverage-html hatch scripts so the main test matrix no longer runs benchmark tests
• Add a benchmarks job to the test workflow that runs benchmarks in parallel with the other test jobs
• test-complete gates on the new benchmarks job

Motivation

Benchmark tests (especially sharded morton indexing) take 15-22s each and dominate wall time when included in the main test suite. Running them as a separate parallel job avoids blocking the test matrix while still catching breakage on every PR. The assumption here is that these tests are not necessary to run on every OS and dependency set.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.91%. Comparing base (dd86ac7) to head (5fefea8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3873   +/-   ##
=======================================
  Coverage   92.91%   92.91%           
=======================================
  Files          86       86           
  Lines       10709    10709           
=======================================
  Hits         9950     9950           
  Misses        759      759           

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