[Fix] Ensure inferred primary_key is a List[str]

[MichelleArk]

Resolves #10983

Problem

Even though our schemas dictate that node.primary_key is a List[str], it is possible for None values to creep into the primary_key if unique or unique_combination_of_columns are not specified correctly and contain null values. This is a violation of our published manifest schema and can lead to parsing issues downstream.

Solution

Do a better job validating the types of generic test kwargs primary key inference is depending on, especially given they may come from other packages like dbt_utils and not be validated at all (unlike our own constraint structures).

This is not a breaking behaviour change -- no project should have been relying on null values in the primary_key as they are semantically useless. If anything it was probably breaking parsing / validation on the consumption end if the unique or unique_combination_of_columns was invalid in niche cases.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 89.13%. Comparing base (0c51985) to head (29ae8e9).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10984      +/-   ##
==========================================
+ Coverage   89.08%   89.13%   +0.04%     
==========================================
  Files         183      183              
  Lines       23580    23580              
==========================================
+ Hits        21006    21017      +11     
+ Misses       2574     2563      -11     

Flag	Coverage Δ
integration	86.49% <100.00%> (+0.11%)	⬆️
unit	62.79% <100.00%> (ø)

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Components	Coverage Δ
Unit Tests	62.79% <100.00%> (ø)
Integration Tests	86.49% <100.00%> (+0.11%)	⬆️