test(irx): cover null and edge cases for compute layer

[yogendra-17]

Pull Request description

#110 added the compute kernels but only tested the happy paths. This adds the missing null, empty, overflow, and type-mismatch tests so we don't regress on them later. Test-only, no source changes: test_compute.py goes from 15 to 32 tests.

Covers: null-skipping aggregations, all-null/empty columns, float min/max, uint64 sum, filtering with null mask slots, null placement in sort, integer divide-by-zero, and silent int overflow.

How to test these changes

• cd packages/irx && uv run pytest tests/test_compute.py -q

• ...

Pull Request checklists

This PR is a:

• bug-fix
• new feature
• maintenance

About this PR:

• it includes tests.
• the tests are executed on CI.
• the tests generate log file(s) (path).
• pre-commit hooks were executed locally.
• this PR requires a project documentation update.

Author's checklist:

• I have reviewed the changes and it contains no misspelling.
• The code is well commented, especially in the parts that contain more
  complexity.
• New and old tests passed locally.

Additional information

Reviewer's checklist

Copy and paste this template for your review's note:

## Reviewer's Checklist

- [ ] I managed to reproduce the problem locally from the `main` branch
- [ ] I managed to test the new changes locally
- [ ] I confirm that the issues mentioned were fixed/resolved .