chore: adding unit tests for offset enum

[yhl25]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.31%. Comparing base (e8017cd) to head (ea4cc7a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/numaflow-core/src/message.rs	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2170      +/-   ##
==========================================
- Coverage   62.57%   62.31%   -0.26%     
==========================================
  Files         324      324              
  Lines       38355    38385      +30     
==========================================
- Hits        24000    23919      -81     
- Misses      13308    13415     +107     
- Partials     1047     1051       +4     

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