fix: validate Python callable file and name together

[cen-zp]

Summary

• validate that python_callable_name and python_callable_file are provided together
• raise a clear DagFactoryException for either incomplete configuration
• cover both missing-pair cases and tighten the existing exception assertion

Root cause

The builder only rejected configurations where all Python callable options were absent. When exactly one file-based callable parameter was provided, it accessed the missing dictionary key directly and raised KeyError.

Validation

• hatch run tests.py3.11-3.2:test — 295 passed, 7 skipped, 1 deselected
• pytest tests/test_dagbuilder.py -q — 87 passed, 4 skipped
• applicable pre-commit checks on the changed files

Fixes #101

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.67%. Comparing base (3c29d9e) to head (9dc5ab2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #849   +/-   ##
=======================================
  Coverage   95.66%   95.67%           
=======================================
  Files          13       13           
  Lines        1385     1387    +2     
=======================================
+ Hits         1325     1327    +2     
  Misses         60       60           

☔ View full report in Codecov by Harness.
📢 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.

[tatiana]

@cen-zp Thanks for the fix, and nice call not just flipping the and to or as the issue suggested - that would have broken the valid "python_callable only" and "name + file" configs. Doing an XOR check to the python_callable-absent case feels like the right approach,

One optional cleanup: the new check repeats not task_params.get("python_callable") right before the existing if not ...:, so it reads as three back-to-back python_callable checks.

I'm happy to approve and merge it once you change it out of draft!

[cen-zp]

Hi @tatiana, thanks again for the review! I’ve applied the optional cleanup and marked the PR ready for review. The latest workflow runs also appear to be awaiting maintainer approval. Could you please take another look when you have a chance?