feat: add support for timestamp_seconds expression

[andygrove]

Summary

• Adds native Comet support for Spark timestamp_seconds function (also known as seconds_to_timestamp)
• Converts seconds since Unix epoch to a timestamp
• Supports Int32, Int64, and Float64 input types

Test Plan

• Added unit tests in CometTemporalExpressionSuite
• Tests include random data, literal values, nulls, negative values (before epoch), and fractional seconds
• All existing tests pass

Note: This PR was generated with AI assistance.

Closes #3111

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.87%. Comparing base (f09f8af) to head (bbfd11e).
⚠️ Report is 848 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3146      +/-   ##
============================================
+ Coverage     56.12%   59.87%   +3.74%     
- Complexity      976     1414     +438     
============================================
  Files           119      168      +49     
  Lines         11743    15588    +3845     
  Branches       2251     2589     +338     
============================================
+ Hits           6591     9333    +2742     
- Misses         4012     4946     +934     
- Partials       1140     1309     +169     

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