GROOVY-11915: GINQ: Add groupby...into with first-class GroupResult type

[paulk-asert]

Summary of changes

New files (2):

• GroupResult.java — interface extending Queryable with getKey() and key(String)
• GroupResultImpl.java — implementation extending QueryableCollection, materializes group elements as a list for reuse

Modified files (6):

• GroupExpression.java — added intoAlias field with getter/setter, updated getText()
• GinqAstBuilder.java — added KW_INTO keyword, dispatch block for into that sets alias on GroupExpression, hedge error for where after groupby...into, reordered conditions so hedge is checked before generic
  DataSourceHolder check
• GinqAstWalker.groovy — added __GROUPBY_INTO_ALIAS metadata key, getGroupByIntoAlias() helper, modified visitGroupExpression to generate groupByInto() call with simple having lambda, modified
  getLambdaParamName/correctVariablesOfLambdaExpression/validateGroupCols to bypass rewriting in into-mode
• Queryable.java — added groupByInto() method signatures
• QueryableCollection.java — added groupByInto() implementation
• ginq-userguide.adoc — added "Group Into" subsection with examples

Tests added:

• 3 backend API tests in QueryableCollectionTest.groovy
• 5 DSL-level tests in GinqTest.groovy (single-key, multi-key, sum, having, multi-key with as)
• 2 error tests in GinqErrorTest.groovy (where-after-into hedge, into-without-groupby)

Key design decisions in implementation:

• GroupResultImpl materializes elements via toList() to avoid stream-already-consumed errors when aggregates are called multiple times (having + select)
• Single-key getKey() unwraps the NamedRecord to return the raw value
• into is parsed as a chained method call (groupby(n).into(g)), not as part of groupby's arguments — cleanly separates key aliasing (as) from group aliasing (into)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.12698% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.4901%. Comparing base (5e07369) to head (c820e64).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...ovy/org/apache/groovy/ginq/dsl/GinqAstBuilder.java	70.5882%	4 Missing and 1 partial ⚠️
...q/provider/collection/runtime/GroupResultImpl.java	66.6667%	1 Missing and 2 partials ⚠️
...he/groovy/ginq/dsl/expression/GroupExpression.java	75.0000%	1 Missing ⚠️
...ovider/collection/runtime/QueryableCollection.java	88.8889%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2453        +/-   ##
==================================================
+ Coverage     66.4799%   66.4901%   +0.0103%     
- Complexity      30238      30269        +31     
==================================================
  Files            1409       1411         +2     
  Lines          117953     118016        +63     
  Branches        20976      20987        +11     
==================================================
+ Hits            78415      78469        +54     
- Misses          33066      33073         +7     
- Partials         6472       6474         +2     

Files with missing lines	Coverage Δ
...oovy/ginq/provider/collection/GinqAstWalker.groovy	90.4161% <100.0000%> (+0.2607%)	⬆️
.../ginq/provider/collection/runtime/GroupResult.java	100.0000% <100.0000%> (ø)
...vy/ginq/provider/collection/runtime/Queryable.java	76.6667% <100.0000%> (+0.8046%)	⬆️
...he/groovy/ginq/dsl/expression/GroupExpression.java	73.3333% <75.0000%> (+0.6061%)	⬆️
...ovider/collection/runtime/QueryableCollection.java	90.0568% <88.8889%> (-0.0306%)	⬇️
...q/provider/collection/runtime/GroupResultImpl.java	66.6667% <66.6667%> (ø)
...ovy/org/apache/groovy/ginq/dsl/GinqAstBuilder.java	88.9344% <70.5882%> (-1.3739%)	⬇️

... and 4 files with indirect coverage changes

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[daniellansun]

Merged. Thanks.