Planner support for VALUES-based BlockJoins

[frouioui]

Description

This PR adds query planning support for the new BlockJoin execution strategy, as proposed in the RFC.

We’ve also streamlined the engine primitive, starting with a minimal implementation before introducing advanced features.

To ensure stability, the new join strategy is currently gated behind a feature flag and query hint—it will only be activated if either the feature flag is enabled or the query explicitly includes the /*vt+ ALLOW_BLOCK_JOIN */ directive.

At this stage, usage is restricted to inner joins involving exactly two standard tables. Further development and testing in the planner will be required to support additional query scenarios.

Related Issue(s)

• Part of Proposal to Optimize Join Operations with Chunking Using VALUES Statement #16508

Documentation

• Add allow_block_join comment directive to the docs website#1962

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 209 lines in your changes missing coverage. Please review.

Project coverage is 67.57%. Comparing base (41a8d1d) to head (4bc5500).
Report is 30 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/planbuilder/operators/block_join.go	40.38%	62 Missing ⚠️
go/vt/vtgate/planbuilder/operators/op_to_ast.go	89.90%	32 Missing ⚠️
.../vt/vtgate/planbuilder/operators/query_planning.go	36.58%	26 Missing ⚠️
go/vt/vtgate/planbuilder/operators/block_build.go	51.51%	16 Missing ⚠️
...o/vt/vtgate/planbuilder/operators/query_builder.go	66.66%	15 Missing ⚠️
go/vt/vtgate/engine/block_join.go	41.66%	14 Missing ⚠️
go/vt/vtgate/planbuilder/operators/phases.go	84.50%	11 Missing ⚠️
go/vt/vtgate/planbuilder/operators/misc_routing.go	23.07%	10 Missing ⚠️
go/vt/vtgate/planbuilder/operator_transformers.go	63.63%	8 Missing ⚠️
go/vt/vtgate/planbuilder/operators/route.go	33.33%	4 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17641      +/-   ##
==========================================
+ Coverage   67.50%   67.57%   +0.06%     
==========================================
  Files        1595     1600       +5     
  Lines      259640   260262     +622     
==========================================
+ Hits       175280   175864     +584     
- Misses      84360    84398      +38     

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