Fix #9593: Validation of unit test parsing for incremental models

[aranke]

resolves #9593

Problem

Unclear error message when is_incremental override wasn't specified for a unit test

Solution

Add parse-time errors when:

• unit testing an incremental model but not providing an override for is_incremental
• unit testing an incremental model overriding is_incremental: true but NOT providing an input for this

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/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

All modified and coverable lines are covered by tests ✅

Project coverage is 88.09%. Comparing base (2c1926c) to head (5fb241e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9769      +/-   ##
==========================================
+ Coverage   88.05%   88.09%   +0.03%     
==========================================
  Files         178      178              
  Lines       22487    22495       +8     
==========================================
+ Hits        19802    19817      +15     
+ Misses       2685     2678       -7     

Flag	Coverage Δ
integration	85.48% <100.00%> (+0.03%)	⬆️
unit	61.79% <0.00%> (-0.03%)	⬇️

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