[UUID 3/8] UUID result rendering (DataSchema, Arrow/JSON encoders)

[xiangfu0]

Parent tracking issue: #16619

What

Renders UUID result columns as canonical lowercase RFC-4122 strings.

Changes

• DataSchema.ColumnDataType.UUID / UUID_ARRAY + conversion and format helpers
• Arrow and JSON broker response encoders
• UUID null placeholder: ColumnDataType.UUID overrides getNullPlaceholder() to return the nil UUID, matching the default null sentinel FieldSpec already uses for UUID columns. Its stored type BYTES supplies a zero-length placeholder, which is not a valid 16-byte UUID and fails to render, so the three call sites that build null-aware blocks (DataBlockBuilder, GroupByResultsBlock, GroupByDataTableReducer) now resolve the placeholder on the logical type rather than the stored type. For every other logical type the two are identical — an invariant pinned by a new DataSchemaTest case.
• DataSchema.fromBytes reports an unrecognized ColumnDataType token with a message naming the mixed-version cause instead of a bare IllegalArgumentException.

Enables

SELECT, GROUP BY, DISTINCT, and join outputs render UUID strings instead of hex.

⚠️ Rolling-upgrade note: DataSchema serializes ColumnDataType by name; an old broker/server that receives a UUID column token from a new node will fail to parse it. Treat as an atomic-upgrade feature — UUID columns should only be queried once the whole cluster is upgraded. No existing (non-UUID) column is affected.

About this PR / how to review

This is part 3 of 8 splitting #18140 (first-class logical UUID type) into layered PRs, as requested there.

The split is enabled by the v1 design: DataType.UUID has stored type BYTES, so most paths handle it automatically; each PR adds explicit UUID semantics to one subsystem. Head branch lives on xiangfu0/pinot.

Both parents (#18869, #18870) have merged, so this PR is now rebased directly on master and the Files-changed tab shows only this layer.

Full stack (merge bottom → top)

1. [UUID 1/8] Add logical UUID type foundation (pinot-spi) #18869 — [UUID 1/8] logical UUID type foundation (pinot-spi) ✅ merged
2. [UUID 2/8] UUID ingest and segment storage #18870 — [UUID 2/8] UUID ingest and segment storage ✅ merged
3. [UUID 3/8] UUID result rendering (DataSchema, Arrow/JSON encoders) #18871 — [UUID 3/8] UUID result rendering (DataSchema, Arrow/JSON encoders) ← this PR
4. [UUID 4/8] Server-side predicate evaluation for the logical UUID type #18872 — [UUID 4/8] UUID server-side predicates, CAST and transforms
5. [UUID 5/8] UUID aggregation, group-by and distinct #18873 — [UUID 5/8] UUID aggregation, group-by and distinct
6. [UUID 6/8] UUID multi-stage engine (planner + runtime) #18874 — [UUID 6/8] UUID multi-stage engine (planner + runtime)
7. [UUID 7/8] UUID partitioning #18875 — [UUID 7/8] UUID scalar UDFs and partitioning
8. [UUID 8/8] UUID integration tests, benchmarks and docs #18876 — [UUID 8/8] UUID integration tests, benchmarks and docs

PRs 4 and 7 (#18872, #18875) depend only on the merged foundation and can be reviewed in parallel; PRs 5/6 (#18873, #18874) also need this one; PR 8 (#18876) needs #18874.

Full feature description, v1 design contract, scope exclusions, and benchmark numbers: #18140.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.11111% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.51%. Comparing base (ab3fd2b) to head (69c8a42).

Files with missing lines	Patch %	Lines
...java/org/apache/pinot/common/utils/DataSchema.java	51.42%	28 Missing and 6 partials ⚠️
...not/core/query/reduce/GroupByDataTableReducer.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18871      +/-   ##
============================================
+ Coverage     65.49%   65.51%   +0.02%     
  Complexity     1421     1421              
============================================
  Files          3428     3428              
  Lines        217518   217587      +69     
  Branches      34558    34570      +12     
============================================
+ Hits         142453   142544      +91     
+ Misses        63527    63508      -19     
+ Partials      11538    11535       -3     

Flag	Coverage Δ
custom-integration1	?
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
java-25	65.51% <61.11%> (+0.02%)	⬆️
temurin	65.51% <61.11%> (+0.02%)	⬆️
unittests	65.50% <61.11%> (+0.02%)	⬆️
unittests1	56.80% <61.11%> (+0.04%)	⬆️
unittests2	37.93% <2.22%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[xiangfu0]

Documentation follow-up: pinot-contrib/pinot-docs#940