fix: populate model constraints regardless of contract enforcement

[MichelleArk]

Resolves #10219

Problem

Solution

When contract.enforced is false, model-level constraints are now
serialized to node.constraints as expected. Invalid constraint types
are filtered out silently (rather than causing a mashumaro error)
when the contract is not enforced.

Adds functional tests covering constraints with enforced: false,
no contract config, and no columns defined.

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.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.25%. Comparing base (f474171) to head (baae080).
⚠️ Report is 64 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12710      +/-   ##
==========================================
- Coverage   91.38%   91.25%   -0.13%     
==========================================
  Files         203      203              
  Lines       25792    25942     +150     
==========================================
+ Hits        23569    23673     +104     
- Misses       2223     2269      +46     

Flag	Coverage Δ
integration	88.05% <62.50%> (-0.15%)	⬇️
unit	65.46% <62.50%> (-0.01%)	⬇️

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Components	Coverage Δ
Unit Tests	65.46% <62.50%> (-0.01%)	⬇️
Integration Tests	88.05% <62.50%> (-0.15%)	⬇️

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