changefeedccl: fix decimal writes into parquet

[KeithCh]

Previously we wrote decimals into parquet as
string bytes. This caused issues when being read
since the readers expect the bytes to be a valid
decimal representation.

Epic: CRDB-41784
Fixes: #130909
Release note (bug fix): Fix decimal writes into parquet files from
changefeeds and exports.

[cockroach-teamcity]

This change is 

[Copilot]

Pull Request Overview

This PR fixes the encoding of decimal values in parquet files by switching from string bytes to a proper twos-complement byte representation. Key changes include:

• Expanding the decimal test case in writer_test.go to cover a broader range of decimals.
• Introducing new helper functions (twosComplement and formatDecimal) in write_functions.go for proper decimal encoding.
• Updating the decimal decoding logic in decoders.go to convert from the twos-complement format.

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

File	Description
pkg/util/parquet/writer_test.go	Updated the test case for decimals by adding more decimal columns and test values.
pkg/util/parquet/write_functions.go	Added functions to compute the twos-complement and format decimals correctly.
pkg/util/parquet/testutils.go	Extended datum validation to include comparisons of decimal coefficients and sign.
pkg/util/parquet/decoders.go	Revised decimal decoding to convert twos-complement bytes to decimal values.

Files not reviewed (1)

• pkg/util/parquet/BUILD.bazel: Language not supported

Comments suppressed due to low confidence (1)

pkg/util/parquet/writer_test.go:240

• [nitpick] The updated test values for decimals now use finite numbers instead of '-inf' and 'nan'. Please verify that these finite values provide sufficient coverage of edge cases previously intended.

if datums[1], err = tree.ParseDDecimal("1.222"); err != nil {

[KeithCh]

The current status is that it works for 95% of cases, with the following caveats:

• We're currently converting all decimal columns into a tuple with columns (decimal, string). But this means we cannot handle when decimals are already in a tuple since we do not support writing to parquet where we have nested tuples. e.g. (decimal, int) -> ((decimal, string), int)
• Exports to parquet do not work when dealing with inf, -inf etc., it causes a panic. Changefeeds are working fine.