[Round 2] Fix #9005: Allow singular tests to be documented in properties.yml

[aranke]

Resolves #9005

Problem

Take 2 of #10744 that was reverted in #10790, accounting for scenario where customer has already used the data_tests YAML key.

Solution

Skip parsing and warn the user that the data_tests key is now used for data test documentation.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

[codecov]

Codecov Report

Attention: Patch coverage is 86.76471% with 9 lines in your changes missing coverage. Please review.

Project coverage is 89.17%. Comparing base (2ff3f20) to head (cdf5c26).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10792      +/-   ##
==========================================
+ Coverage   89.11%   89.17%   +0.06%     
==========================================
  Files         183      183              
  Lines       23288    23456     +168     
==========================================
+ Hits        20752    20916     +164     
- Misses       2536     2540       +4     

Flag	Coverage Δ
integration	86.41% <86.76%> (+0.09%)	⬆️
unit	62.09% <41.17%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	62.09% <41.17%> (-0.11%)	⬇️
Integration Tests	86.41% <86.76%> (+0.09%)	⬆️

[gshank]

This looks good, but I do wonder if we might want to give a more detailed explanation in the warning, mentioning that "data_tests" is a newly parsed key? Or something...