chore: update CI workflows for improved packaging and testing

[ElouenGinat]

Add CI workflow and improve pip packaging tests

Summary

This PR introduces a dedicated CI workflow for testing the Python source code and improves the wheel testing step in the pip packaging workflow.

Changes

New: .github/workflows/ci.yml

• Adds a CI workflow triggered on pushes to main and on pull requests touching ci.yml, pyproject.toml, src/**, or tests/**
• lint job: runs pre-commit hooks on all files
• test job: runs the test suite against the source code across Python 3.10–3.14 using uv sync --group dev

Updated: pack-pip.yml

• Add src/** to the path filter so packaging is triggered when source files change
• Add a concurrency group to cancel in-progress runs on new pushes
• Replace bare pip usage with uv for isolated, reproducible wheel testing:
  • Create a dedicated venv (.test-env)
  • Install the built wheel with all extras (khisto[all])
  • Install dev dependencies from pyproject.toml via uv sync --group dev --no-install-project (consistent with CI)
  • Run pytest without coverage to keep the check fast
• Upgrade actions/download-artifact from v5 to v6
• Minor formatting fixes (indentation, whitespace)