UFix64 <-> UInt256 conversion fix and init test coverage

[sisyphusSmiling]

Closes: #56

Description

Fixes UFix64 <-> UInt256 conversion bug that simply truncated decimal values and adds test coverage for the util functionality.

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For contributor use:

• Targeted PR against main branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.89%. Comparing base (428634e) to head (ebb0683).
Report is 72 commits behind head on main.

Files	Patch %	Lines
cadence/contracts/bridge/FlowEVMBridgeUtils.cdc	96.15%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   82.88%   85.89%   +3.00%     
==========================================
  Files          18       19       +1     
  Lines         818      886      +68     
==========================================
+ Hits          678      761      +83     
+ Misses        140      125      -15     

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