Preserve identifiers across to-one joins

[XX-Q]

Summary

• keep outer collection identifiers when a to-one join has a keyless inner source
• add regression coverage for sync/async single-query and split-query materialization

When the inner side of a cardinality-preserving join has no identifiers, clearing the outer identifiers loses the collection element identity. Single-query materialization then collapses rows, while split-query materialization indexes an empty comparer list. A to-one join cannot increase cardinality, so the existing outer identifiers remain sufficient.

Fixes #38660

Validation

• Regression test with the source fix reverted: 4/4 cases fail with the reported missing-element and ArgumentOutOfRangeException behaviors
• Focused regression test with the fix: 4 passed
• AdHocQuerySplittingQuerySqliteTest: 21 passed
• EFCore.Relational.Tests: 1,480 passed, 1 skipped
• Full EFCore.Sqlite.FunctionalTests: 37,917 passed, 283 skipped
• ./build.sh --build: passed with 0 warnings and 0 errors
• Original standalone reproduction: all single/split and keyed/keyless comparison cases return both elements after the fix

SQL Server functional tests were not run locally because no SQL Server instance is available; the shared relational test will exercise the SQL Server provider in CI.

Checklist

• I have read the contributing guidelines and walkthrough
• I posted the implementation plan and validation evidence on the issue; the fix follows the maintainer-confirmed direction
• The code builds and tests pass locally
• The commit message follows the repository format and includes Fixes #38660
• Regression tests have been added
• The code follows existing patterns and style

AI assistance

OpenAI Codex assisted with implementation and test execution. The final diff was reviewed against the maintainer-provided root cause and validated with the commands above.

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[Copilot]

Pull request overview

Fixes a relational query materialization bug where a cardinality-preserving (to-one) join with a keyless inner source could incorrectly clear outer collection identifiers, leading to silently missing collection elements in single-query mode and an ArgumentOutOfRangeException in split-query mode. The change updates identifier handling during join composition and adds a regression test covering sync/async and single/split query execution.

Changes:

• Adjust join identifier propagation logic in SelectExpression.AddJoin to preserve outer identifiers for to-one joins even when the inner side has no identifiers.
• Add a new regression test reproducing the keyless set-operation inner source scenario and validating correct collection materialization across sync/async and single/split query modes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/EFCore.Relational.Specification.Tests/Query/AdHocQuerySplittingQueryTestBase.cs	Adds regression coverage for the keyless-inner to-one join scenario across sync/async and single/split query materialization.
src/EFCore.Relational/Query/SqlExpressions/SelectExpression.cs	Updates join identifier clearing logic to avoid dropping outer collection identifiers for to-one joins with keyless inner sources.

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[AndriySvyryd]

You need to disable value generation for Order.Id

[XX-Q]

Fixed in 0f8a433: Order.Id now uses ValueGeneratedNever(), and TaxA/TaxB no longer seed explicit identity values. The focused 4-case regression test and the full 21-test SQLite class both pass locally.