[codex] funcadl uproot rntuple parity

[gordonwatts]

What changed

• Refactored the func_adl uproot ROOT writer to use a shared helper for root-file and root-rntuple output.
• Added tests for the writer contract and an empty-array root-rntuple transform path.

Why

• This brings the func_adl generator in line with the raw uproot generator’s output behavior, especially for RNTuple handling.

Validation

• Local execution of Python tests was not possible in this environment because only the Windows Store python.exe shim is available here.
• Changes were reviewed statically and committed as a focused two-file patch.

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.48%. Comparing base (1ae268b) to head (032315c).
⚠️ Report is 20 commits behind head on develop.

Files with missing lines	Patch %	Lines
..._code_generator/templates/transform_single_file.py	89.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1496      +/-   ##
===========================================
+ Coverage    88.94%   89.48%   +0.54%     
===========================================
  Files          103      104       +1     
  Lines         3689     3958     +269     
  Branches       453      513      +60     
===========================================
+ Hits          3281     3542     +261     
- Misses         333      337       +4     
- Partials        75       79       +4     

☔ 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.

[BenGalewsky]

@gordonwatts do you want this in the 1.8 release? The clock is ticking...