feat: add partial support for date_format expression

[andygrove]

Summary

• Adds native Comet support for Spark's date_format function
• Uses DataFusion's to_char function for the underlying implementation
• Supports a whitelist of common format strings that can be reliably mapped between Spark SimpleDateFormat patterns and strftime patterns

Supported formats include:

• Date formats: yyyy-MM-dd, yyyy/MM/dd, yyyyMMdd, yyyyMM
• Time formats: HH:mm:ss, HH:mm, HH, mm, ss
• DateTime formats: yyyy-MM-dd HH:mm:ss, yyyy/MM/dd HH:mm:ss
• Day/month names: EEEE, EEE, MMMM, MMM
• 12-hour time: hh:mm:ss a, hh:mm a, h:mm a
• ISO format: yyyy-MM-dd'T'HH:mm:ss
• Single components: yyyy, yy, MM, dd

Unsupported format strings will fall back to Spark execution.

Timezone Support

Currently only UTC timezone is fully compatible. Non-UTC timezones are marked as Incompatible and fall back to Spark by default. Users can enable Comet execution for non-UTC timezones with spark.comet.expr.DateFormatClass.allowIncompatible=true, but results may differ from Spark.

See #3202 for tracking full timezone support.

Test Plan

• Added comprehensive tests in CometTemporalExpressionSuite:
  • Tests all supported format strings with timestamp columns
  • Tests with literal timestamps (constant folding disabled)
  • Tests null handling
  • Tests fallback to Spark for unsupported formats
  • Tests non-UTC timezone fallback behavior
  • Tests allowIncompatible config for non-UTC timezones

Note: This PR was generated with AI assistance.

Closes #3088

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.20690% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.00%. Comparing base (f09f8af) to head (8d8e544).
⚠️ Report is 851 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/main/scala/org/apache/comet/serde/datetime.scala	85.96%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3201      +/-   ##
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+ Coverage     56.12%   60.00%   +3.88%     
- Complexity      976     1429     +453     
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  Files           119      170      +51     
  Lines         11743    15746    +4003     
  Branches       2251     2603     +352     
============================================
+ Hits           6591     9449    +2858     
- Misses         4012     4977     +965     
- Partials       1140     1320     +180     

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