Add experimental openCypher graph query frontend

[xiangfu0]

Summary

Adds an experimental graph query feature that translates a subset of openCypher into SQL JOINs and executes via Pinot's multi-stage engine. This enables graph-style traversal queries over existing Pinot tables without native graph runtime operators.

• New pinot-graph module (SPI + planner) with hand-written Cypher parser → IR → SQL transpiler
• POST /query/graph broker endpoint behind pinot.graph.enabled feature flag (default: false)
• Supports 1-hop and 2-hop traversals, WHERE (comparisons, AND/OR/NOT, STARTS WITH, ENDS WITH, CONTAINS, IN), RETURN (DISTINCT, AS aliases, COUNT/SUM/AVG/MIN/MAX with GROUP BY), ORDER BY, SKIP, LIMIT
• Graph schema passed inline in request JSON mapping vertex/edge labels to Pinot tables
• Zero impact on existing SQL query path — purely additive changes

Design doc posted on #17935: #17935 (comment)

Closes #17935

Test plan

• 211 unit tests (parser: 94, SQL generator: 49, translator: 52, golden: 16) — all pass
• 12 broker handler tests — all pass
• 10 E2E integration tests with 2 servers and distributed data (50 users, 200 edges, 4 segments/table) — all pass
• spotless, checkstyle, license checks — all clean
• Existing SQL path unaffected (no modifications to query execution)
• Feature disabled by default; returns clear 400 error when disabled
• Unsupported Cypher syntax rejected with clear error messages

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.80228% with 213 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.50%. Comparing base (2892236) to head (99acfad).

Files with missing lines	Patch %	Lines
...a/org/apache/pinot/graph/planner/CypherParser.java	88.41%	30 Missing and 27 partials ⚠️
...che/pinot/graph/planner/CypherToSqlTranslator.java	72.32%	27 Missing and 17 partials ⚠️
...pinot/broker/api/resources/PinotClientRequest.java	0.00%	43 Missing ⚠️
...a/org/apache/pinot/graph/planner/SqlGenerator.java	85.50%	27 Missing and 12 partials ⚠️
...e/pinot/graph/planner/CypherSemanticValidator.java	79.77%	9 Missing and 9 partials ⚠️
.../java/org/apache/pinot/graph/planner/CypherIR.java	97.00%	3 Missing and 2 partials ⚠️
...not/broker/requesthandler/GraphRequestHandler.java	93.75%	0 Missing and 4 partials ⚠️
...ache/pinot/graph/planner/CypherParseException.java	50.00%	2 Missing ⚠️
.../org/apache/pinot/graph/spi/GraphSchemaConfig.java	96.42%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17938      +/-   ##
============================================
+ Coverage     65.39%   65.50%   +0.11%     
+ Complexity     1405     1399       -6     
============================================
  Files          3423     3431       +8     
  Lines        215968   217283    +1315     
  Branches      34186    34382     +196     
============================================
+ Hits         141228   142333    +1105     
- Misses        63380    63520     +140     
- Partials      11360    11430      +70     

Flag	Coverage Δ
custom-integration1	?
integration	?
integration1	?
integration2	?
java-25	65.50% <83.80%> (+0.11%)	⬆️
temurin	65.50% <83.80%> (+0.11%)	⬆️
unittests	65.50% <83.80%> (+0.11%)	⬆️
unittests1	57.12% <ø> (+<0.01%)	⬆️
unittests2	37.86% <83.80%> (+0.28%)	⬆️

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