feat: Use FirstOrDefaultAsync for contract retrieval

[kamilbaczek]

Replaces SingleOrDefaultAsync with FirstOrDefaultAsync when fetching contracts by customer ID. This change prevents exceptions if multiple contracts exist for a customer and returns the most recently prepared contract.

[Copilot]

Pull Request Overview

This PR updates contract retrieval logic to handle scenarios where multiple contracts exist for a single customer. The change replaces SingleOrDefaultAsync with FirstOrDefaultAsync to prevent exceptions and return the most recently prepared contract based on the existing OrderByDescending sort.

• Changes contract query method to use FirstOrDefaultAsync instead of SingleOrDefaultAsync

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.55%. Comparing base (24f85a7) to head (9dfa0f5).
Report is 373 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #209      +/-   ##
==========================================
+ Coverage   96.49%   96.55%   +0.05%     
==========================================
  Files          61       62       +1     
  Lines         542      551       +9     
  Branches        9        9              
==========================================
+ Hits          523      532       +9     
  Misses         16       16              
  Partials        3        3              

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