Fix data branch decimal results on main

[gouhongshen]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #24847

What this PR does / why we need it:

Port the data branch decimal result conversion fix to main.

• Preserve decimal precision/scale metadata from MySQL result columns and validate invalid/missing decimal metadata.
• Materialize decimal64/decimal128/decimal256 row values as typed MatrixOne vectors when building data branch output batches.
• Decode and compare decimal256 raw tuple values in data branch hashdiff/change replay paths.
• Add focused unit tests and BVT coverage with diff_14.sql for decimal64/decimal128/decimal256 data branch diff.

Notes:

• The pick subquery string-literal fix from 565a0f71 is already present in main with tree.WithSingleQuoteString() and unit coverage.
• The decimal256 PK pick issue is tracked separately in [Bug]: data branch pick does not support decimal256 primary keys #24902 and intentionally not fixed in this PR.

Checks:

• make -C thirdparties
• source $HOME/.zshrc && moenv && go test ./pkg/container/types ./pkg/frontend ./pkg/vm/engine/disttae/logtailreplay
• git diff --check upstream/main...HEAD

Draft until CI confirms this port on main.

[gouhongshen]

Updated this PR to address the review coverage gaps.

Covered in this push:

• Added focused appendFromEntry() coverage for decimal256, including null handling.
• Expanded diff_14 to cover decimal UPDATE / INSERT / DELETE paths under save_query_result=on.
• Added a product-level pick_12 conflict case using decimal(39,4) as a non-PK value, covering fail / skip / accept behavior. This also covers the decimal256 pick output path without mixing in the separately tracked decimal256 PK issue ([Bug]: data branch pick does not support decimal256 primary keys #24902).

Local validation on main:

• source $HOME/.zshrc && moenv && go test ./pkg/frontend ./pkg/vm/engine/disttae/logtailreplay
• BVT diff_14: 15/15 passed
• BVT pick_12: 69/69 passed
• git diff --check

Re-requesting review.

[gouhongshen]

已把 3.0-dev 上处理 change request 的 decimal256 data branch pick 修复 port 到 main。

本次更新 commit: 4259e88572 fix: support decimal256 data branch pick keys

本地验证：

source $HOME/.zshrc && cd /Users/ghs-mo/MOWorkSpace/matrixone-2nd && moenv && cd /private/tmp/matrixone-port-decimal-main && go test ./pkg/container/types ./pkg/container/vector ./pkg/frontend ./pkg/sql/colexec/lockop ./pkg/vm/engine/readutil ./pkg/objectio/mergeutil ./pkg/vm/engine/tae/mergesort -run 'TestSimpleTupleAllTypes|TestSingleTypeTuple|TestDecimal256OrderTuple|TestTupleSQLStrings|TestTupleSQLStringsWithEncodeDecode|TestDecodeTupleWithSchema|TestUnpackNthElement|TestType_DescString|TestFetchDecimal256Rows|TestFetchDecimal256RowsWithFilter|TestFetchDecimal256RowsWithFilterAll|TestNumericGap_AllTypes|TestIsNumericType|TestAppendNumericStringToVec_AllTypes|TestCoercePickKeyVectorToType_Decimal256ReformatsToTargetType|TestExtractPKValDecimal256|TestFormatPickKeyVectorValueAsString_AllSupportedKinds|TestLCAProbeJoinCastType|TestConstructBlockPKFilter|TestMergeBaseFilterInKind|TestMergeSortBatchesDecimal256|TestAObjMergeDecimal256' -count=1

结果：PASS。git diff --check 也已通过。