Implement LINQ GroupJoin() operator for cross-document joins

[jeremydmiller]

Summary

• Adds LINQ GroupJoin() support for cross-document-type SQL JOINs between Marten document tables
• GroupJoin + SelectMany translates to INNER JOIN
• GroupJoin + SelectMany + DefaultIfEmpty translates to LEFT JOIN
• Uses CTE-based SQL generation so each document table retains the standard d alias, then joins the CTEs with a jsonb_build_object projection
• Supports duplicated fields in both join keys and projections (columns are included in CTEs via SelectClauseWithDuplicatedFields)
• Supports Count(), First(), and other aggregation operators after joins
• Unsupported patterns (GroupJoin as final operator, composite keys) throw NotSupportedException with clear messages

New files

File	Purpose
src/Marten/Linq/GroupJoinData.cs	Data carrier for parsed GroupJoin expressions
src/Marten/Linq/Parsing/Operators/GroupJoinOperator.cs	Parses GroupJoin expression tree, detects DefaultIfEmpty for LEFT JOIN
src/Marten/Linq/SqlGeneration/JoinSelectClause.cs	ISelectClause rendering JOIN SQL between CTEs
src/Marten/Linq/Parsing/JoinSelectParser.cs	Builds jsonb_build_object projection across two joined collections
src/LinqTests/Operators/group_join_operator.cs	16 integration tests
docs/documents/querying/linq/group-join.md	VitePress documentation with examples

Modified files

File	Change
CollectionUsage.cs	Added GroupJoinData and SelectManyCallExpression properties
CollectionUsage.Compilation.cs	Added CompileGroupJoin() and GroupJoinData check in compileNext()
OperatorLibrary.cs	Registered GroupJoinOperator
SelectManyOperator.cs	Detects GroupJoin source, stores full expression for DefaultIfEmpty detection
LinqQueryParser.cs	DocumentTypes() yields inner types from GroupJoinData
docs/.vitepress/config.mts	Added nav entry for GroupJoin docs

Test plan

• 16 integration tests covering:
  • Inner join (basic, outer-only projection, anonymous type projection, string field join)
  • Left join (DefaultIfEmpty)
  • Unsupported pattern throws NotSupportedException
  • Count and First aggregation after join
  • Duplicated inner key, outer key, both keys, Guid key, projected fields, Count with duplicated key
• Full LinqTests suite (1216 tests) passes with 0 regressions
• Builds cleanly with 0 errors on net9.0

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