fix: update stats serialization logic for scale-0 decimals

[DrakeLin]

Description

Creating Delta tables with Decimal(precision, scale=0) columns fails since in stats code, we store decimals as f64. serde_json converts f64 values like 1234.0 to 1234.0 for scale=0. Arrow Parser then rejects it since it expects 1234.

Modified StatsScalar enum to preserve scale information:

• Changed from Decimal(f64) to Decimal { value: f64, scale: i32 }
• Updated serialization logic to output integers for scale=0 decimal

Tested by building delta-rs and running the example script linked in the issue.

Related Issue(s)

delta-io/delta-kernel-rs#1431

Documentation

[github-actions]

ACTION NEEDED

delta-rs follows the Conventional Commits specification for release automation.

The PR title and description are used as the merge commit message. Please update your PR title and description to match the specification.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.89%. Comparing base (2e5f7b0) to head (9814ab2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3916   +/-   ##
=======================================
  Coverage   73.88%   73.89%           
=======================================
  Files         152      152           
  Lines       39426    39443   +17     
  Branches    39426    39443   +17     
=======================================
+ Hits        29131    29147   +16     
- Misses       8971     8973    +2     
+ Partials     1324     1323    -1     

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[rtyler]

@DrakeLin is this independent of any kernel changes, or is this pending something in kernel to be released?

[DrakeLin]

@rtyler This is independent of any kernel changes. The bug seen in delta-io/delta-kernel-rs#1431 didn't turn out to be a kernel issue (delta-rs doesn't use kernel for writes), so putting up a quick fix in delta-rs. The bug, as the original author pointed out, is due to an issue in serde_json.

This bug caused delta-rs to write stats with scale 0 out as 1234.0. The kernel issue we saw was due to kernel trying to read this and failing. I think there is a kernel fix we can push out as well to relax our constraints on read, but that's independent of this change.